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At least 50 arrests have been made and three police officers were injured during an anti-capitalist protest Thursday in central London.
Thousands of demonstrators wearing Guy Fawkes masks took part in the Million Mask March, which began at Trafalgar Square before winding down to Whitehall.
Campaigners of all ages chanted in small groups under the watch of Metropolitan Police officers - the force had earlier warned about the possibility of violence.
Many carried placards with the message "One solution: Revolution", with others screaming "Whose streets? Our streets" as tourists looked on and took pictures.
The protest started peacefully, but it soon turned ugly, with injured demonstrators being tended to by emergency crews as police and mounted officers formed human shields on sections of Westminster closed off to surging protesters.
Some managed to break free from the main route, heading instead to Buckingham Palace and Green Park, where protesters threw missiles such as cones and fireworks at police horses. One animal bolted as an explosive whizzed past its head.
Later people were seen crowding around a police car which had been set on fire.
Police began threatening people with arrest soon after as the march was officially due to end at 9pm. It went on for at least an hour and a half longer.
Three officers were taken to hospital with injuries caused during the protests - the injuries are not life-threatening.
Met commander BJ Harrington said: "Officers have been hospitalized, a police horse suffered injuries and a police car was criminally damaged during the course of the protest which is completely unacceptable.
"I praise the professionalism and restraint that officers have shown in the face of hostile provocation and enabled the policing operation to pass without major incidents.
"We will bring those responsible for the criminality to face justice."
The worldwide annual event is organized by the anarchic hacking collective Anonymous.
A Facebook page for the London protest said before the march got under way: "The police are not your friends, this goes for police liaison officers too, they're simply a 'friendly face' who collect intelligence.
"Keep an eye out for your comrades and police tactics that will limit movement, the hive mind should stay vigilant."
Nearly 20,000 people said on Facebook they would attend the protest.
Last year, 10 people were arrested amid violence towards officers and public buildings being vandalized.