Hundreds of masked Anonymous protesters descended on Buckingham Palace and Parliament Square as part of a worldwide march against austerity cuts.
A fire was started just yards from the gates of the palace as protesters threw glass bottles during clashes with police last night.
Nelson’s Column in Trafalgar Square and the Victoria Memorial near the palace were damaged during the demonstrations.
A fire was started near the Victoria Memorial, where at least one man and woman were seen being taken away by officers.
Fireworks were also reportedly thrown towards Buckingham Palace.
Eleven people were arrested, Scotland Yard said, nine for public order offences and two for criminal damage.
Officers in riot gear tried to shepherd the crowd, which was part of a pre-arranged Anonymous Million Mask March.
Several similar events were carried out in cities in Japan, Australia, and New Zealand in opposition to austerity cuts, corruption and an increase in state surveillance.
Ceylan Hassan, 24, a university graduate, who was among the protesters in London, 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, travelled to the protest with his brother and friend from King’s Lynn, Norfolk yesterday afternoon.
He 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.
Green MP Caroline Lucas said on Twitter that she had joined the protest against austerity outside Parliament earlier in the day, while comedian Russell Brand was also pictured at demonstrations near Downing Street.