The much-anticipated trouble broke out at Euston station in central London, where a crowd began attacking police and destroying bus shelters. At one point, rioters overturned and set fire to an empty police van - to roars of approval from bystanders.
The violence, which began shortly after 6.30pm, came after a day during which protesters had supported campaigners protesting at the World Trade Organisation summit in Seattle in the United States, by marching through the streets of London and distributing leaflets condemning the WTO.
The well-publicised day of demonstration had started at 9.00am when just a handful gathered at Euston Station before starting a ramshackle march around the capital. However, by early evening the crowd had swollen to several hundred, overflowing the concourse of the station. The campaigners were initially happy to listen speeches and dance to drummers. But then a number began to incite others to violence. "Will you pledge to kill a pig," shouted one speaker.
The mood appeared to change and at around 6.30pm the crowd, now estimated by police to be 750-strong, moved out of the station and confronted a line of uniformed police who were trying to turn them back. It was at this point that demonstrators began attacking officers using placards and hurling cans.
Rapidly, police chiefs ordered in riot officers who arrived at the scene within seconds, equipped with shields, helmets and batons. A series of running fights with demonstrators ensued, as police attempted to move them back with charges.
Just how many of the original demonstrators where involved in the violence was unclear, but there seemed to be a hardcore of no more than 100 who were bolstered and cheered by hundreds of onlookers.
Having confronted the rioters, police withdrew and surrounded the demonstrators who congregated beneath buildings outside the station. Then an empty police van was overturned. Its windows were smashed and attempts made to set it alight using burning placards. Onlookers chanted "Burn it, burn it" and cheered as one man carrying a flaming sign eventually managed to ignite the van, which exploded.
Riot police encircled the rioters, hemming them in to a small area in front of the station where "snatch squads" of officers grabbed people they had identified in the crowd. As the evening wore, on people were only allowed to leave the cordon after they had been searched and photographed. Scotland Yard said 15 people were arrested for public-order offences while seven people were hurt, including one police officer who suffered head and neck injuries.
There had long been fears that yesterday's planned demonstration would end in violence, repeating the summer's incidents in the City which erupted into mayhem and caused millions of pounds worth of damage.
As a result, many City institutions had organised special security measures. News International, publishers of The Times and The Sun newspapers, had sealed off their Wapping headquarters and hired 420 extra security guards against possible attack.
The policing of yesterday's demonstrations was the first time a new joint operation involving three forces was put into action. Set up in the aftermath of June's riots, Operation Benbow was co-ordinated by a 50-strong team of officers based at police headquarters where officers from the Metropolitan Police, the City of London Police, and British Transport Police, followed events on video monitors.