London (AFP) - 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.
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.
Police said 50 people were arrested for offences including criminal damage and police assault and that some protesters were "intent on criminality".
"Officers have been hospitalised, a police horse suffered injuries and a police car was criminally damaged during the course of the protest which is completely unacceptable," Metropolitan Police Commander BJ Harrington said.
"We will bring those responsible for the criminality to face justice."
An annual anti-establishment protest that takes place on Britain's Bonfire Night, the march featured crowds of people wearing white masks intended to emulate Guy Fawkes, now associated with the international Anonymous network.
Fawkes attempted to blow up parliament in 1605 to install a Catholic monarch, and Bonfire Night is commemorated annually to mark the failed "Gunpowder Plot".
"We're here as a collective to speak out for the unspoken people against the corruption, the lack of privacy, the cuts to the poorer people," one anonymous masked protester told AFP.
"The government at the moment are not representing us," said another.
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 windows and engaging in running scuffles with police as shops closed their doors.
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.
A 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 got pushed in the back by people trying to move forward," said Terry Small, 20, from the southern English city of Plymouth. "I couldn't move."
Police had put in place special security measures ahead of the march, which also saw scuffles and arrests last year, and formed cordons surrounding two groups of marchers in Trafalgar Square and outside parliament.
Under the measures, demonstrators were to be confined to certain areas of central London and were required to remove their masks if asked by police and to disperse by 2100 GMT.
Three men "acting suspiciously" were arrested before the demonstration got under way, and were found to be in possession of knives, lock picks and smoke grenades, police said.