Demonstrations against rallies by right-wing extremists have led to the arrest of 58 anti-racism campaigners amid angry scenes in central London.
Police invoked public order powers to detain the activists as they attempted to block supporters of the British National Party and English Defence League from marching on the Cenotaph war memorial.
A hardcore of around 50 far-right sympathisers had gathered outside parliament on Saturday, amid heightened tensions over the death of Lee Rigby in Woolwich.
Tempers flared as they were met by a crowd of more than 300 anti-extremist campaigners allied with groups including Unite Against Fascism and Hope Not Hate, who attempted to face them down in a counter-demonstration.
Dozens of police attempted to keep the opposing groups apart, with sniffer dogs deployed in an effort to subdue the crowds.
Scotland Yard said the anti-racist activists spilled out of a pre-arranged pen area as skirmishes broke out between rival groups.
With clashes erupting across Westminster, a police spokesman said: "Due to police concerns about serious disruption to the life of the community, and the potential for serious disorder should this counter protest confront the BNP organised protest, police have imposed conditions under Section 14 of the Public Order Act.
"Those conditions state that the protest must take place in Whitehall Gardens junction with Whitehall. A group of about 300, also believed to be part of the UAF protest, were stopped in Old Palace Yard junction with Abdingdon Street.
"This group have now been notified of the conditions imposed under Section 14 and requested to move to Whitehall Gardens to continue their protest. Officers are in negotiation with this group."
BNP leader Nick Griffin, whose plan to march on a mosque in Woolwich was vetoed by police, addressed the crowd shortly after lunch, with his supporters roused by the sound of the national anthem.
The BNP leader said Rigby's murder was not an "isolated" incident, and added: "We're pointing out that it will happen again and again and again until the West disengages with Islam and they leave our country."
Rigby's family had appealed for calm in advance of the protests, saying they could not abide the prospect of violence in their son's name.
Arrests were also made in Leeds after members of the English Defence League clashed with groups opposed to their extreme nationalist ideology.
Witnesses said running fights broke out following disturbances during the planned minute's silence to honour the murdered soldier.
In Sheffield, a 100-strong mob tried to storm a wreath-laying ceremony in memory of Drummer Rigby. EDL groups also gathered in Wakefield, Rotherham, Halifax and Doncaster.
The majority of those arrested are thought to have been anti-extremist campaigners.
Adding to the confusion, hundreds of animal welfare activists in fancy dress had also converged on Westminster to protest against the government's cull on badgers.