At least three police officers are in hospital after being injured during an anti-capitalist march.
The Metropolitan Police said 28 arrests had been made by 10.40pm, 100 minutes after the Million Mask March was officially due to ended in the capital.
The protest, organised to hit back at austerity measures and perceived inequality brought about by government, started peacefully enough, with songs and chants at Trafalgar Square.
But the march soon turned ugly, with bloodied bodies 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.
A Met statement said: "To prevent further crime and disorder, two containments are in place in the vicinity of St Martin's Place, Trafalgar Square and Parliament Street, Whitehall.
"At this stage, 28 arrests have been made, most for public order offences.
"Three officers have been taken to hospital with injuries sustained policing the protests, the injuries sustained are not believed to be life-threatening.
"A man has been arrested for assaulting a police officer in connection with one of the incidents where an officer was injured. The man is in custody at central London police station."