A man was being questioned today after gangs of youths attacked police officers following an anti-racist march, Scotland Yard said.
Cordons were set up to stop people going up Whitechapel Road, in east London, because officers feared random attacks on members of the public.
The scenes followed a rally, organised by Unite Against Fascism, that attracted several thousand people.
It was organised in response to another rally planned by the far-right English Defence League (EDL) which was called off earlier this week.
A police spokesman said yesterday's UAF rally was "well organised and well stewarded" but a group of young men gathered outside the nearby East London Mosque in response to rumours the EDL were 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.
"Police officers were attacked by the crowd at points throughout the afternoon. One member of the public was attacked at random by members of the crowd as those gathered surged up and down the Whitechapel Road.
"In order to prevent injuries to the public and officers, and serious disorder, police withdrew from the immediate area and a series of filter cordons were put in place. The cordons were used to prevent access to parts of Whitechapel Road due to concerted efforts by the crowd to attack people at random."
The cordon was in place for two hours.
One person was arrested for assault and was being held for questioning.
A spokesman for Unite Against Fascism said around 5,000 took part in the march from Stepney to Whitechapel.
He said: "I heard there were a few nasty scuffles between local youths and police but certainly the demo was very positive and a really good vibe."