"Items including bricks and bottles have been thrown at officers," said a spokesman for London's Metropolitan Police amid a tense stand-off as the sun set over Harrow Central Mosque.
One person was arrested to prevent a breach of the peace, and nine others for possession of offensive weapons including bottles of bleach, a hammer and a chisel, he added.
Police moved in after a crowd of angry Muslim youths, some wearing masks over their faces, threw sticks and stones at a small group of about a dozen mostly shaven-headed protestors.
"This is England, I should be able to demonstrate," said one of the anti-Islamic protestors.
"I have got two sons in the army. They are out in Afghanistan fighting, but the police doesn't (sic) want to defend us here today," said the man in his late 40s, who declined to give his name.
In a tense atmosphere after initial clashes subsided, police surrounded the white demonstrators, to shield them from the angry Muslims some 500 metres (yards) from the mosque.
The demonstration was organised by Stop Islamification of Europe, which said ahead of the demo that it planned to remain peaceful.
Stephen Gash of the SIOE -- whose motto is "Racism is the lowest form of human stupidity, but Islamophobia is the height of common sense" -- said before the demo: "We don't want any more mosques until all this hatred is sorted out."
But the violence erupted when they were confronted by hundreds of protestors who the police said were either members of a group called United Against Fascism, or Muslims providing support to the mosque.
At one point hundreds of Muslim protestors surged towards a man who they believed was a member of the anti-Islam protest, as he ran in the direction of police lines.
It appeared that the man was struck several times before police pushed the Muslim protestors back.
Khairul Khan, 34, said Muslims were not being aggressive.
"We are just defending our own mosque. We are doing nothing wrong, we are just trying to stop them attacking the house of our God," he said.
Muslim protestor Abdullah Al-Andalusi, 30, said: "Anyone should be allowed to demonstrate all they want, but if you are inciting hatred against any human being, then you should not be allowed to do that.
"And under English law you can't do that."
Concerns about violence have been heightened by clashes last weekend at a rally against Islamic fundamentalism held by a right-wing group, the English Defence League.
More than 30 people were arrested in Birmingham, Britain's second city, when the demonstrators fought with anti-fascist campaigners.
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