Scores of supporters of the English Defence League threw bottles at police and chanted anti-Muslim slogans in Woolwich hours after the murder of one man and the shooting of his two suspected assailants.
About 100 men, including some wearing balaclavas printed with "EDL", engaged in running battles with police for less than an hour.
A police commander said officers cited section 60 of the Public Order Act, which allowed them to stop and search individuals within a specific area without evidence of a crime being committed.
EDL leader Tommy Robinson said: "They're chopping our soldiers' heads off. This is Islam. That's what we've seen today. They've cut off one of our army's heads off on the streets of London.
"Our next generation are being taught through schools that Islam is a religion of peace. It's not. It never has been. What you saw today is Islam. Everyone's had enough. There has to be a reaction, for the government to listen, for the police to listen, to understand how angry this British public are."
The men congregated near Woolwich Arsenal station with flags bearing the cross of St George after plans for a protest were circulated on social media. Hundreds of police including riot police were stationed around Woolwich.
The first clashed began just after ten before the men were pursued and blocked in at a pub close to a local mosque.
Elsewhere, two men were arrested after separate attacks on mosques. A 43-year-old man is in custody on suspicion of attempted arson after reportedly walking into a mosque with a knife in Braintree, Essex. The secretary of the mosque, Sikander Saleemy, told Channel 4 News he felt it was a "revenge attack".
He added: "The police said it's too early to try and link it to what happened in Woolwich, but those of us who were here feel that it was some sort of revenge attack. It was clear from the man's behaviour.
"We absolutely condemn what happened in Woolwich, but it had nothing to do with us. It was an appalling act of terror – but it wasn't Islamic in any way. I wish it wasn't described like that, because sadly people will now start to blame Muslims."
Meanwhile, police in Kent were called to reports of criminal damage at a mosque in Canterbury Street, Gillingham, at 8.40pm. A spokesman said a man was in custody on suspicion of racially aggravated criminal damage.