Gangs of youths attacked police officers after an anti-racist march in east London, Scotland Yard said.
Cordons were set up to stop people going up Whitechapel Road because officers feared random attacks on members of the public.
The scenes followed a march, organised by Unite Against Fascism (UAF), that attracted several thousand people.
This article was prepared by the Islam in Europe blog - islamineurope.blogspot.com
It was in response to a rally planned by the English Defence League (EDL), but called off earlier this week.
A police spokesman said the UAF rally had been "well organised and well stewarded" but a group of young men gathered outside the nearby East London Mosque in response to rumours the EDL was planning a protest.
He said: "The group numbered up to 300, who were very volatile.
"Despite continued excellent attempts by stewards and representatives from the East London Mosque to control the crowds, even placing themselves in danger, there was the risk of serious disorder.