Activists set a police car alight and scuffled with riot police in central London as thousands took part in a "Million Mask March" anti-capitalist demonstration.
Many activists wore the white masks associated with the international Anonymous network at the march, an annual anti-establishment protest that takes place on Britain's Guy Fawkes night.
Demonstrators threw fireworks and bottles at police, some of whom were knocked from their horses, while several bleeding protesters were treated for injuries as authorities tried to contain the march on Thursday night.
Three men "acting suspiciously" were arrested before the demonstration got underway and were found to be in possession of knives, lock picks and smoke grenades, police said.
Protesters chanted "One solution: revolution" and "Whose streets? Our streets" and split into different factions, confronting lines of police outside Buckingham Palace and the office of Prime Minister David Cameron.
Some demonstrators ran through London's major shopping district around Oxford Street, smashing window and engaging in running scuffles with police as shops closed their doors.
One protester who emerged from the crowd with blood pouring from his head close to the headquarters of the ruling Conservative party said he had been caught between a surge of demonstrators and police batons.
"I was at the front, I was chatting with the officers asking them what their agenda was and why they were trying to stop us walking down the street," said Terry Small, 20, from the southern English city of Plymouth.
"I got pushed in the back by people trying to move forward... I couldn't move."
One group of activists gathered outside the British premiere of the finale of the Hunger Games in Leicester Square, as stars Jennifer Lawrence and Julianne Moore walked the red carpet.
Mexican anarchist Aztecarna Peatonito said the march was a protest against inequality.
"We tried for six years to get the press to take us seriously. It is only when we wore the masks that we started getting attention," Peatonito said.
"Anyone who is violent is not with us."
Police had put in place special security measures ahead of the march, which also saw scuffles and arrests last year.
Under the measures demonstrators were to be confined to certain areas of central London and to be required to remove their masks if asked by police.