Hundreds of Anonymous protesters wearing Guy Fawkes masks protested outside Buckingham Palace, Queen Elizabeth II's London home, as part of a global demonstration against austerity.
Protesters threw bottles at police in riot gear and a fire was lit outside the palace.
"They started shouting move back, move back, but we had nowhere to go," said protester Ceylan Hassan.
"The police started pushing us, screaming 'move back, move back'.
"There was a fire on the right hand side of the (Victoria) monument and people started throwing things," she added.
The demonstration was part of a pre-arranged "million mask" protest by "hacktivist" group Anonymous with similar events taking place in Japan, Australia, and New Zealand.
The protest coincided with Bonfire Night in Britain, the annual commemoration of the failed 1605 "Gunpowder Plot", in which Guy Fawkes attempted to blow up parliament in order to install a Catholic monarch.
The mask of his face, as popularised in the comic book series V for Vendetta, has come to symbolise the international protest movement against austerity and capitalist excesses.
