She unfurled a red and yellow banner with Chinese lettering . . . the symbols of the Falun Gong. Evil people die early, " she screamed in Chinese. Hu, your days are numbered." Then, in English, she addressed the leader of the US: President Bush, stop him from persecuting the Falun Gong! Stop him from killing! Stop the torture and killings! Falun Dafa is good."

As the two presidents looked on stunned, and the Chinese leader tried falteringly to continue his speech, she railed on. It was almost three minutes before she was bundled away by embarrassed US officials who began an immediate inquiry as to how she had got so close to the world leaders.

Members of Falun Gong pop up wherever Chinese leaders travel, but they are usually kept well away, outside the concentric security cordons. But the woman, Wang Wenyi, had been granted an official pass as a reporter for the Epoch Times, a Chinese newspaper which denies it is a front for the Falun Gong but tends to be remarkably sympathetic to it. If anyone had checked, they would have found that Wang, then a pathologist, had heckled Hu's predecessor, Jiang Zemin, five years ago.

So why exactly is this? After all, Falun Gong seems nothing more than a version of the early morning calisthenics which can be seen being practised in parks in cities across the planet, in which groups of people glide in unison through a set of tranquil ritualised movements known as qigong . . . a form of exercise which, like yoga, can be both physical and spiritual. It is a technique that involves controlled breathing and five sets of meditation exercises which, only a few years after its public introduction in 1992, quickly grew to become one of the most popular forms of qigong in Chinese history.

Why then, is it perceived by the government in Beijing to be the most subversive threat to the Chinese state in the six decades since the Communist Revolution?

Falun Gong was developed and registered in 1992 by a former army musician and clerical worker, Li Hongzhi, in Manchuria.

Amid the physical exercises developed the unshakeable belief that Li was a 'Living Buddha' who had supernatural powers and had rediscovered the basic law (fa) of the universe. Officially, the movement is known as Falun Dafa but it is more usually known as the Falun Gong. In his writings, Li made use of many concepts from Buddhism, Taoism and Christianity which gave Falun Gong the feel of a fully-fledged religion.

The movement found easy converts among ordinary Chinese because of its focus on improving health in a country where years of underinvestment have put the Chinese health system under severe strain, and resulted in a return to traditional methods of healing. A craze for Falun Gong swept the country in the early 1990s, attracting up to 70 million adherents. In those early years, Chinese governmental organisations granted several awards to Li to encourage him to continue promoting what they then considered a wholesome practice.

Things changed in 1999, after a magazine printed an article by a physicist which was critical of Falun Gong. Practitioners went to protest outside the publishers and several were arrested and, apparently, beaten by the police. Two weeks later, 10,000 of Li's followers suddenly assembled around Zhongnanhai, the high-security complex that houses China's leaders. They held no signs and chanted no slogans but sat in a twokilometres line in meditative postures on the pavements. The demonstration was peaceful and ended quietly after 12 hours with the protesters picking up their litter and dispersing of their own accord, but only after an audience with Premier Zhu Rongji and a promise that the group's grievances would be addressed within three days.

It was the largest organised show of opposition since the Tiananmen democracy movement a decade earlier. Such a large and devastatingly disciplined demonstration struck fear into the hearts of the Communist Party and its leader, Jiang Zemin. If Falun Gong really had 70 million members, then it was bigger than the party itself, which had 66 million. He resolved to smash the organisation. Two months later the practice of Falun Gong was outlawed.

Some of Li's claims are pretty wacky. He says he can levitate and become invisible simply by thinking the phrase ?nobody can see me". He claims to have averted a global comet catastrophe and the Third World War, and says that Nostradamus's prophecies are coming true today. In the real world his impact is mixed. Falun Gong inculcates an 'us and them' feeling among its followers.

But it has health benefits for millions, with studies showing that its exercises reduce stress and may boost the immune system.

Nor does Li seem to have milked its enthusiasts. The worst financial scandals critics have uncovered are that, between 1992 and 1994 in China, he collected modest fees for treating patients. And, according to the Wall Street Journal, he purchased a house in New York for $293,500 in 1998, and another for $580,000 in New Jersey in 1999. Falun Gong has no church buildings, rented spaces, priests or administrators. And Li has been nominated twice for the Nobel peace prize.

The crackdown in China began roughly.

Companies were ordered to dismiss Falun Gong members. Many were taken into police custody, beaten and told to give up the practice. Of those who refused, around 100,000 were sentenced to ?re-education through labour" programmes. At least 500 have been sentenced to up to 18 years in prison, 100 of whom are thought to be in Jinlin Jail.

Uncounted numbers were forcibly admitted into mental hospitals. Falun Gong claims there are 2,840 cases of deaths in custody.

Huge pressure is placed on local officials to comply with central edicts. Not only do they face no scrutiny for the methods they use to eliminate the Falun Dafa, they are now personally fined by Beijing for every protester from their district who arrives in the capital. Out of this culture has grown the practice, claimed in a report by supporters outside China, in which detainees have had their internal organs harvested . . . while they were still alive . . . for sale on the black market for heart, kidney, liver and cornea transplants.

External support for these claims has come in the form of the US state department's annual human rights report, which says that, since the introduction of the ban on Falun Gong, the mere belief in the discipline. . . has been sufficient grounds for practitioners to receive punishment ranging from loss of employment to imprisonment". Some practitioners in custody have suffered torture and death, it says, and hundreds have been punished without trial.

The extraordinary thing is that, in the face of this, Falun Gong members have persisted in their protests. Supporters have hijacked Chinese TV stations to broadcast protests. They have demonstrated regularly in Hong Kong. And they have continued the exercises out of sight of the authorities.

China's leaders have not lost their fear. It was the late Zhou Enlai who once famously said, when asked what were the effects of the French Revolution, it's too soon to tell".

History is a present reality to the men at the top of the party, for all their embrace of the free market. Many of the Falun Gong adherents, they believe, are disillusioned members of the party from its Maoist days who need an outlet for the zeal which has not featured in political life since the end of the Cultural Revolution. Worst of all, China's leaders remember that religious sects can grow in power and turn into national rebellions.

In the 19th century, the Taiping Rebellion sprang from a religious cult which provoked perhaps the bloodiest civil war in history when the forces of the Qing Empire clashed with those of a mystic named Hong Xiuquan who said he was the younger brother of Christ and claimed to be the new Messiah.

At least 20 million people perished.

Not long after that came the Boxer Rebellion, in which rebels also saw the world in more metaphysical terms, claiming that movement exercises influence the fundamental forces of the universe. They even insisted that their breathing exercises would allow them to ward off bullets. No wonder the normally inscrutable Hu looked taken aback when Wang Wenyi screamed.