Lozells Road, north-west Birmingham, on a bustling Saturday afternoon in honeyed sunshine: Asian women wheel trolleys of food out of Bangla Station supermarket or peruse the racks in Majitex, "for all your textile needs". Afro-Caribbean men share their motoring problems with Asian staff over the counter in the Jivaji auto parts store. Children of all colours emerge from Sadiq's kebab house, and heads of all colours are groomed at Poppy's salon, ready for the weekend's revelry. It is as normal a British urban high-street scene as one could imagine.

But round the corner, in Villa Road, every window of the Asian Resource Centre is shattered - a reminder that a few weeks ago, this landscape of multicultural normality was the stage for a vicious riot that left scores injured, property ransacked, a police officer shot in the leg and a young black passerby, with no connection to the violence, stabbed to death. A riot with a new twist: a stand-off between Afro-Caribbeans and Asians. "We have never had black-on-Asian violence," says Martin Blisset, the appalled chairman of the Afro-Caribbean Millennium Centre. "It is something new and terrible, which shows how our society is changing. We seem to be destroying what is best about this country - that people of different cultures learn to live together."

The disturbances are said to have started with a rumour that went through innumerable mutations - essentially, that a black teenager was gang-raped in an Asian-owned hair and beauty shop called Beauty Queen on Wellington Road, near Lozells.

Rumour is nothing new in the history of race riots. As far back as 1943, the Harlem riots were detonated by news on the street that a white policeman had killed a black soldier (there had been a confrontation, but no fatality). The Detroit riot a few weeks earlier was ostensibly sparked by twin rumours at the ghetto's edge: that a black woman and her baby had been killed in a park and, on the other side of the tracks, that black men had raped and killed a white woman on the park bridge.

But though it has precedents, the Birmingham rape rumour is a peculiarly modern tale. It is a story about the shifting terrain of British race relations and about the terrifying momentum that a whisper - whatever relationship it may bear to the truth - can acquire in a world of pirate radio and the internet.