Suddenly she has begun to look her age. Aung San Suu Kyi was nearly 45 when her party won a landslide victory in Burma's historic general election of 1990, but she looked 15 years younger. Despite years of privation and isolation inside her disintegrating lakeside villa on University Avenue in Rangoon, she continued to look far younger than she was. But last November, when she was photographed arriving for a meeting at a Rangoon hotel with President Obama's assistant secretary of state Kurt Campbell, she looked a woman of a certain age.

Daw Aung San Suu Kyi - daw is the Burmese honorific for an older woman - turned 65 on 19 June. If it was a less dismal occasion than her 64th, which she spent in Rangoon's huge, British-built Insein Prison, while awaiting trial for allowing a deluded American called John Yettaw (who had swum across Lake Inya using home-made wooden flippers) to spend two nights in her home, her personal and political prospects were scarcely less gloomy.

The good news: the military regime, led by Senior General Than Shwe, is committed to holding a general election before the end of the year. It will be the first since 1990, when Aung San Suu Kyi's party, the National League for Democracy (NLD), won the vote by an overwhelming margin - a victory that the regime ignored. The bad news: there is no chance of that result being repeated this time. The NLD, the most important organised opposition force in Burma, will not be participating.

One of the rules of this election is that parties whose membership includes political prisoners are barred from registering. The NLD was faced with the obligation of expelling its leader and more than 400 members who are still in prison. Aung San Suu Kyi made it clear that taking part on such terms was unthinkable. She wanted party members to know that, should they participate in the election, "the party would have no dignity", her lawyer and spokesman Nyan Win said. After a painful debate, the NLD agreed with her assessment. When the 6 May deadline passed, it became a non-party, a political phantom.

Since her victory in Burma's first multiparty elections in 30 years in 1990, and particularly since she was awarded the Nobel Peace Prize the following year, Aung San Suu Kyi has become, to the west, the symbol of opposition. Now the caprice of the regime has robbed her of that automatic primacy. Inside Burma, too, many are said to be puzzled and confused by the NLD's decision not to compete.

Like the constitutional referendum of 2008, which the regime claimed to have won with the support of more than 90 per cent of voters, the election will probably be rigged. But that does not mean it is without significance. "Despite the very obvious flaws in the process," says the international think tank Transnational Institute in a new report anticipating the poll, "it represents the most significant political transformation for a generation."

Senior General Than Shwe, now almost 80, is likely to retire, along with other high-ranking colleagues. "New leaders and a new political landscape will emerge," the institute writes, "giving rise to opportunities to press for change, as well as a new set of challenges."

It is tempting to see Aung San Suu Kyi as one of the elders who will now shuffle off into the shadows, to be replaced by vigorous new champions of democracy, the generational shift sweeping her away as surely as her nemesis Than Shwe. But this is to misunderstand the situation in Burma. First, the electoral process has been fiendishly designed to make it almost impossible for figures critical of the regime, even if they succeed in getting elected, to make their voices heard. There is no provision in the parliament for an opposition - only for the government, which must be led by a former army man. The election will not be accompanied by any loosening of the rigid controls on the media; on the contrary, the regime has recently invested millions of dollars in a hi-tech system for censoring newspapers and magazines. It will be a criminal offence, punishable by a jail sentence, for any MP in the new national assembly to criticise the constitution - and anything he or she says on the subject will be erased from the record. The regime clearly has no intention of allowing new MPs the sort of holiday from government control that Aung San Suu Kyi and her colleagues enjoyed during the democratic spring of 1988-89.

The second reason she will not be fading away is that Aung San Suu Kyi remains an immensely potent force. That is why the regime has gone to such extravagant lengths to marginalise her. Three times before - in 1990, 1995 and 2002 - it made the mistake of under­estimating her appeal. The first time, the NLD humiliated the regime's proxy, the National Unity Party, by winning 80 per cent of the seats in parliament. The second time, when she was released from her first spell of house arrest, thousands risked jail every week to squat outside the gates of her home and listen to her speak. The third time, when, after months of delay, she was at last allowed to travel outside Rangoon, peasants walked through the jungle for days for a chance to see her.

Seven years have now passed since her convoy was attacked by regime goons on 30 May 2003 and she again disappeared from view. Her latest spell in detention is the longest to date, and in March the UN's working group on arbitrary detention condemned it as being in contravention of Articles 9, 10, 19 and 20 of the Universal Declaration of Human Rights - the sixth time it has published such a view.

This has been not only the longest spell of house arrest she has endured, but also the most restrictive. The regime has rigidly limited her access to foreign diplomats, senior members of her party and, for a while, even her doctor. A year ago, Ban Ki-moon, the UN secretary general, was barred from seeing her at Insein jail. She has been photographed only a handful of times in the past seven years, most recently on 10 May, when she and Assistant Secretary of State Campbell held a conference under umbrellas in the garden of a government guest house to avoid having their conversation bugged. As recently as 2007, rebel monks succeeded in paying a visit to her home, but today the whole of University Avenue is barricaded off, and the only view of her house is from the other side of Lake Inya.

The tight control on her comings and goings is matched by a ban on all but the most dismissive and hostile mentions of her in the state-controlled press. The time in 1994 when she was welcomed amid the gleaming varnish and antimacassars of a government reception room by Generals Than Shwe and Khin Nyunt, both smiling fit to bust (the photo was splashed across the front page of New Light of Myanmar), seems to belong to an altogether different era.

Aung San Suu Kyi seems to have disappeared from the consciousness of ordinary Burmese people. There are no photos of her on display; there are no books by or about her in the bookshops (though titles about Nelson Mandela and other democratic heroes are plentiful); her name is never mentioned by tour guides or taxi drivers or hotel staff. It is as if she had never existed. However, she has not been forgotten, but is locked away in people's hearts and minds and interior rooms as securely and invisibly as she is locked in her own house.

At the home of an intellectual in Mandalay, an oil portrait of her hangs on the wall of his shabby kitchen. Another presides over a small private library in the same city that has somehow escaped the regime's attentions. Htein Lin, an artist who spent years in jail and who now lives in exile in London, contrived to paint portraits of her secretly in his cell and get them smuggled to the outside world. In Rangoon, we were given an introduction to a beaming elf of a man in his seventies, living with his family in a shack near a rubbish dump on the city's northern outskirts. He had spent years in jail for petty political offences, as had his son. A small photo of Aung San Suu Kyi in her 1989 prime was pinned to the wall of their home. It is not surprising that Suu the Unyielding should continue to be the icon of activists. But her appeal extends more widely, as became clear during the nationwide rebellion of September 2007 when, in reaction to a steep increase in fuel prices, tens of thousands of monks marched through Burma's towns and cities.

In the most poignant of these processions, a line of monks persuaded soldiers guarding Aung San Suu Kyi's house to let them through. They made it to the gate of her home; she came out in tears to greet them. The meeting was captured on a mobile-phone camera. Two days later, the bloody crackdown began.

The encounter had a special significance that was not readily appreciated outside Burma. In this overwhelmingly Buddhist country, the great majority of boys are inducted as monks while they are still children, and spend weeks or months learning the disciplines of the monastery. For centuries, the monks have had a special relationship with power in the land, and specifically with the king: by recognising his right to rule, they conferred religious legitimacy on him, while for his part the king provided funds for monasteries, pagodas and images of the Buddha. This symbiotic relationship was destroyed when the British sent Burma's last king, Thibaw Min, into exile in 1885, which helps explain why monks were prominent among Burmese rebels against British rule from early on. It was they, deprived of patronage, who most keenly felt the downfall of the monarchy.

Burma's post-independence prime minister, U Nu, was a pious Buddhist and was quick to restore the relationship with the clergy, but when General Ne Win seized power in a coup d'état in 1962, he brushed aside the Sangha, the organisation of monks, as an archaic irrelevance. Into the patronage vacuum stepped pious laypeople who, over the years, set up meditation centres and supported charismatic monks who, in return, instructed them in dharma (the teachings of the Buddha) and the techniques of meditation. By the time Ne Win realised what he had done, the lay meditation organisations had established strong ties with the monks - ties that persist to this day. On the face of it, there is nothing political about these organisations; but just as the Sangha, numbering close on half a million monks and nuns, is the only organisation in Burma that rivals the army, the existence of these lay organisations is a passive but ominous menace to the army's monopoly on power.

Aung San Suu Kyi has taken her own daily meditation practice very seriously since her first years of house arrest. "It has helped to strength­en me spiritually," she told Alan Clements, author of a book of interviews with her entitled The Voice of Hope. "What you do when you meditate is you learn to control your mind through developing awareness." She confided to a friend that meditation had "saved me from depression in the worst moments of my life . . . It's what enabled me always to hold my head high." So when, in 2007, the monks came to her gate to greet her, it was an acknowledgement by the Sangha of the power that she had won through the ballot box in 1990 but of which she had been deprived ever since. And millions of Burmese would have recognised the significance of the meeting.

With its innumerable spies, the regime is surely well informed about the extent to which Aung San Suu Kyi still enjoys the silent but overwhelming support of her people, which is why it remains loath to grant her any space. But how, despite nearly 15 years of house arrest and enforced silence and invisibility, has she been able to hang on to this support? And what difference has it made?

In the first place, it was necessary for her to be who she is: the daughter of Aung San, the father of the army and creator of independent Burma, the young firebrand who managed to hop from the Japanese to the Allied side in the middle of the Second World War and, at the end of it, persuade Lord Mountbatten that he was his nation's best hope. He negotiated Burma's independence in London but was assassinated in 1947 before his work could be crowned with success - and was thereby untainted by all the mess that followed, becoming the new nation's one radiant, immaculate hero.

At the start, in 1988, the name was everything: if Aung San Suu Kyi's elder brother Aung San Oo, an engineer who lives in the US, had chosen to rise to the challenge, he would have enjoyed just as strong a following as she did, but he had no interest in politics. As with Benazir in Pakistan, Indira in India and all the other widows and daughters in post-colonial southern and south-east Asia, the blood was crucial. It guaranteed popular legitimacy and gave the uprising its figurehead.

Yet she became much more than that, and is still there - still fighting - over 20 years later. This is the product of her own extraordinary strength of will and purpose. Her name and fame have so far deterred the regime from killing her, though on at least two occasions
it seems to have come close to it. At any time since she was first locked up, she could have decided to put her own life and that of her family first and fled the country, never to return. Instead, she threw herself into Burma's struggle, and for all the regime's efforts to thrust her into the shadowy margins, she is still in the thick of it. Because, to quote her hero Gandhi: "The combat itself is the victory."

So, what difference has she made? A generation ago, the regime's opponents saw their only hope of changing the country in joining one or other of the insurgencies raging on the borders. Taking up arms was seen as the only option. Aung San Suu Kyi's courage and commitment have changed that. For the first time since Ne Win's 1962 coup d'état, Burma had a plausible alternative ruler - and one who insisted that the struggle against the regime must be a peaceful one.

Her critics argue that this commitment condemns her movement to failure: non-violence may have enjoyed some success as a strategy against the Raj in India, but it is bound to fail against a regime as ruthless and as little concerned about its foreign reputation as Burma's. In response, her supporters point out that the revolution she is seeking to ignite is as much moral and spiritual as political. And if it has not yet had any appreciable softening effect on the generals, it has changed the attitudes of a generation of Burmese activists.

As long as she is still there among them, and still fighting, it gives them hope.

Peter Popham is on the staff of the Independent and is writing a biography of Aung San Suu Kyi, to be published by Rider in 2011.