Comedian Russell Brand joined hundreds of masked Anonymous protesters in central London as part of a worldwide demonstration against austerity cuts.
Fireworks were reportedly thrown towards Buckingham Palace and a fire was started next to the nearby Victoria Memorial, while some of those present clashed with police.
Glass bottles were flung at officers during the disorder and 11 people were arrested, nine for public order offences and two for criminal damage.
Police in riot gear tried to shepherd the crowd, who also gathered in Parliament Square as part of the pre-arranged Anonymous Million Mask March.
A number of similar demonstrations took place in cities in Japan, Australia, and New Zealand in opposition to austerity cuts, corruption and an increase in state surveillance.
Brand was pictured alongside fellow protesters in London, wearing one of the trademark masks adopted by hacktivist group Anonymous.
He later wrote on his Twitter page: “Whatever party they claim to represent in the day, at night they show their true colours and all go to the same party #MillionMaskMarch.”
Brand’s comments follow an appearance on Newsnight and a guest editorship of the New Statesman in which he called for a “total revolution of consciousness and our entire social, political and economic system”.
Others involved in the London demonstration accused the police of heavy-handed tactics.
Ceylan Hassan, 24, a university graduate, said: “They started shouting move back, move back, but we had nowhere to go. The police started pushing us, screaming ‘move back, move back’.
“There was a fire on the right hand side of the monument and people started throwing things.”
Protesters were moved back away from Buckingham Palace as the atmosphere grew tense.
Some of those involved in the march were seen ripping barriers from the side of the road.
Sean Roesner, 21, a self-employed computer programmer at the protest, said: “We turned up and the protest was at Buckingham Palace. When we arrived people were firing fireworks at the palace.
“It was funny. I didn’t have any but I would have fired some if I had.
“I joined Anonymous because I was arrested under the Computer Misuse Act. I spent eight months on police bail last year and had done nothing wrong.
“We are here to stand up for what we believe in, to make the world a better place.”
Mr Roesner said his friend had been among a group of people encircled by the police on The Mall.