Tensions have been simmering in central London as the English Defence League (EDL) marched to Downing Street.
About 1,000 protesters chanted "Muslim killers off our streets" and "There's only one Lee Rigby" in tribute to the soldier killed in Woolwich, south-east London, last Wednesday.
A massive police presence kept them separate from a smaller group of anti-fascist activists, with officers making 13 arrests in total for a range of public-order offences. EDL leader Tommy Robinson told demonstrators outside Downing Street: "This is a day of respect for our Armed Forces."
The crowd repeatedly chanted "coward" after he claimed Prime Minister David Cameron was on holiday in Ibiza, Spain, "because he doesn't care". Mr Robinson added: "They've had their Arab Spring. This is time for the English Spring."
Whitehall was closed to traffic during the protest, which saw EDL members carry placards with slogans that read "Blood on your hands" and "GB RIP". Meanwhile, groups such as Unite Against Fascism chanted "Fascist scum off our streets" and "Follow your leader, kill yourself like Adolf Hitler".
Parmajeet Singh, who is part of a Sikh vigil camped in front of Downing Street against human rights abuses in India, explained how the group got caught in the middle as both sets of protesters threw glass bottles at each other.
Mr Singh, 50, said: "One of them fell in our crowd but did not hit anybody and we were not going to respond. I saw Tommy Robinson telling his crowd to stop. We did have women and a pram in here, and the police were very good, they stopped it quickly."
Police intervened again when about 100 anti-fascists mobbed five or six men who were arriving late to the protest draped in Union Jacks and wearing England football shirts. The anti-fascist group followed and heckled the EDL supporters until officers formed a cordon between them.
EDL sympathisers congregated after their march in Leicester Square, central London, and as a youth was taken away by police some members of the crowd threw missiles, hitting one officer on the head with a beer bottle.
Skirmishes broke out around the square, with several men threatening a youth wearing a Palestinian scarf before police intervened. More EDL supporters were arrested after they made their way back to Downing Street before being dispersed by police.