Rhythm of a revolution: The rock band that changed the world

They're described, a little fancifully, as "the only rock group to have altered the course of European history." Nobody seems able to describe the actual sound they make, although helpful reviewers have called it "free jazz," "driving rock 'n' roll" and "like a raging klezmer orchestra fronted by Frank Zappa."

Once unknown outside their native Czechoslovakia, they play a vital part in Tom Stoppard's play Rock 'n' Roll, the smash-hit of last summer that traces the history of political upheaval in their birthplace from 1968 to 1989. And two weeks from now, you'll be able to experience them, live and cooking, when they play the Queen Elizabeth Hall in London's South Bank.

They're The Plastic People of the Universe (named after a line from a Frank Zappa song,) also known as The Plastics or PPU, and the gig will commemorate the 30th anniversary of the founding of Charter 77, the human rights petition that paved the way for the overthrow of Communism in 1989.

But when the band perform, they'll be shouldering a considerable burden - not just because this is their first gig in the UK, but because they'll wish to be seen as more than a footnote in history, a purely symbolic blip on the radar of world revolution.

God knows they have suffered for their art in the past. They've been arrested, purged, banned from playing, their leaders imprisoned, even deported, but they came back for more. And now they're anxious to escape the past, where they lie trapped in their iconic status like flies in amber, and to re-establish themselves as genuine musicians.

Who are these guys? They were formed in September 1968, just a month after the libertarian Prague Spring of Alexander Dubcek was crushed by Russian tanks. A bassist called Milan Hlavsa was their musical leader, a fan of Lou Reed's Velvet Underground, the Doors, Captain Beefheart, the Fugs and Zappa's Mothers of Invention. They were joined a year later by a charismatic cultural historian called Ivan Jirous, who brought with him a lead guitarist Josef Janicek, and a John Cale-style viola player called Jiri Kabes.

Like Pink Floyd and Soft Machine in the UK, their concerts were groovy "happenings," with psychedelic lighting and ill-advised costumes. Photographs from the 1960s show a quintet of wispy visionaries in neo-classical togas, playing in a sylvan glade and looking faintly idiotic.

As the Kremlin pursued a programme of "normalisation" to re-establish decent Soviet cultural values, some rock bands strategically changed their look to something more acceptable. The Plastics clung to their shaggy, psychedelic integrity and, in the 1970s, lost their professional license and their right to perform for money.

For years, they could play only on the underground circuit. Banned from playing in Prague, they retreated to the countryside but then (on the grounds that their music was "morbid" and would have a "negative social impact") were banned from playing in public at all. They performed in ever-more remote halls in deepest Bohemia, the address of the gig passed on by word-of-mouth. The police sometimes intercepted the whispers and raided the halls, sometimes clubbing and arresting the student audience.

The personnel altered, as did the band's raison d'être. An unusual signing was Paul Wilson, a Canadian graduate from Oxford who had been teaching in Prague since 1967. Jirous employed him to teach the band English lyrics, then asked him to be lead singer. He performed with them at 15 gigs between 1970 and 1972.

"When I played with them," says Wilson now, "the PPU were doing almost exclusively cover tunes by the Velvet Underground and the Fugs, both New York bands but both very much on the fringes of American popular music. There was also a smattering of original work, but even there I translated the texts into English, and we sang them in English - a bizarre arrangement, but English was thought to be the lingua franca of rock music, certainly of underground music as Ivan Jirous understood it.

"The period I played with them was formative, when they really learned how to play together and developed a sound. But there was another tendency - the urge to do original music, performed in Czech. I encouraged it, because it always seemed absurd to me to be singing to an audience that didn't know what we were saying. I effectively did myself out of a job, because I couldn't sing in Czech.

When I left, Vratislav Brabenec (the third "original" member of the current line-up) joined. He was adamantly against performing in English and in favour of doing all original work."

The band began to use the lyrics of Egon Bondy, a surreal comic Czech poet. But that seemingly nationalistic shift didn't please the authorities.

Wilson said: "When I played with them, pressure from the police increased, along with the band's following. Ironically, performing in Czech, rather than gaining them Brownie points with the regime, only increased the hostility, because now audiences could understand what they were singing, and the lyrics they chose were by banned poets, the most maudit being Bondy, whose work had never been published legally.

"As our audiences grew, so did the interest of the police. The band attracted a growing number of frustrated musicians who took heart from its example and started their own bands. The regime ended up facing, not just a growing body of fans, but a mushrooming underground music scene." Wilson left the band in 1972, and returned to Canada.

It was in 1976, after a music festival, that Vaclav Havel met the band. The Czech secret police had arrested 27 musicians and kicked Paul Wilson out of the country. In September, two members of the Plastics, Vratislav Brabanec and Ivan Jirous, appeared on trial, were found guilty of "organised disturbance of the peace" and given prison sentences.

During the trial, Havel, a cutting-edge playwright, met Jirous and resolved to support them. He was at the centre of a group of Czech intellectuals and academics, who set up a human rights organisation. On 1 January, they launched a manifesto and called their organisation after it - Charter 77, the document that kickstarted the chain of events which led, 12 years later, to the Velvet Revolution and the elevation of Havel to be the first president of the Czech Republic.

Havel explained, in his essay The Power of the Powerless, what happened at the trial and why he felt it important to retaliate: "Everyone understands that an attack on the Czech musical underground was an attack on the most elementary and important thing, something that bound everyone together - it was an attack on the very notion of 'living with the truth,' on the real aims of life.

"The freedom to play rock music was understood as a human freedom and thus as essentially the same as the freedom to engage in philosophical and political reflection, the freedom to write, to express and defend the social and political interests of society."

When I met Vaclav Havel last summer, at the launch of Rock 'n' Roll, I asked him if the band were good as a band, quite apart from their symbolic function.

"Yes, they were good," said Havel. "Unlike most rock bands, they had a special mystical flavour about them, a very Prague flavour. It was partly to do with the texts of Egon Bondy and partly with the figure of Milan Hlavsa [the founder and composer.] The fact that the Plastics became the subject of a famous story is another matter - but they couldn't have become the centre of that story without being an interesting band in their own right."

He helped them out on many occasions, letting them use his country home in Hradecek for music festivals, with the police standing in a sullen cordon outside, and releasing tapes of their concert in the West. The band went on making original music through the 1980s. Their last album was Midnight Mouse in 1986, and they finally split in 1988, with the core musicians reconstituted as Pulnoc, meaning "midnight." In 1997, on the 20th anniversary of Charter 77, Havel suggested they should reunite. They did, and have been touring ever since - despite the death in 2001 of Milan Hlavsa.

Tom Stoppard is at pains not to overemphasise the significance of The Plastics. "Let's get this clear," he told The Sunday Times sternly last summer, "The Plastic People did not bring down Communism." Their fascination for him lies in their very indifference to politics, to the passionate activism that surged around them. "They never set out to be symbols of resistance," said Stoppard. "In the West, bands love to be perceived as engaged and politically motivated and don't mind at all if the press writes about their protest rather than their music. But the Plastic People resented this. They wanted to be appreciated for their work."

This, he thinks, is what attracted Havel. "Thanks to the band, intellectuals came to realise that 'living in truth' could take the form of attending a rock concert."

There is, therefore, a lot riding on the Plastics' return to life at the South Bank. The three old-school Plastics, Brabanac, Janicek and Kabes, now refer to themselves as "the Central Committee," and play exclusively Czech music from the 1973-1980 period. Paul Wilson, their old friend and singer, has heard them. "They now play better, and have better equipment, and different instrumentation," he said wistfully.