Media playback is unsupported on your device Media caption Many of the protesters on the Million Mask March were wearing the "Guido" mask

Three police officers have been injured and treated in hospital after clashes with anti-capitalist protesters.

Crowds gathered in Parliament Square, in central London, for Thursday's Million Mask March, many wearing the "Guido" masks of the Anonymous group.

Mounted officers formed a barrier against a densely packed crowd on The Mall while, away from the main rally, a police car was set on fire.

At least 50 arrests were made, mainly for public order offences.

Trafalgar Square arrests

Earlier, London's Met Police said three men - aged 38, 50 and 55 - had been arrested in Trafalgar Square on suspicion of being in possession of offensive weapons.

Police say they found knives, smoke and paint grenades, gas canisters and lock picks.

The Met said three officers sustained injuries while policing the march, but they were not life-threatening. Two people were arrested for assaulting a police officer.

One officer, who was riding a police horse, suffered a fractured wrist. Six police horses were injured during the clashes, says Scotland Yard.

Some protesters were treated for injuries and were given first aid at the scene.

Image copyright EPA Image caption There was a large police presence in central London

The Million Mask March was organised by Anonymous to hit back at austerity measures and perceived inequality brought about by the government.

The protest had started in Trafalgar Square.

Eyewitnesses described a sudden surge of activity about half an hour into the event, as people at the front of the protest broke away and ran towards the Churchill War Rooms.

Crowds gathered outside Buckingham Palace, a distance away from the agreed route.

Some tried to enter Great George Street - a short distance from Conservative Party headquarters - which had been closed off.

Image copyright Getty Images Image caption A police car was attacked and set alight in Queen Anne's Gate, central London

Bottles were thrown over the barriers towards officers and fireworks and flares were set off.

Police said the car that was set alight was parked in a side street and was unattended when it was attacked.

"This was an isolated incident with a small number of people involved, away from the main protests," the Met said in a statement.

'We are peaceful'

One activist, who gave her name as Lola, said she had travelled from Hull to the capital to speak up for others.

"There are people who aren't represented; they should be here," she said.

"The media twist it. We are peaceful, we are doing this in a peaceful manner, but the media focus on the 1% who cause trouble."

Image copyright EPA Image caption Police say the attack on the police car was an isolated incident

Image copyright Getty Images Image caption Three men - aged 38, 50 and 55 - were arrested in Trafalgar Square on suspicion of having weapons

Image copyright AFP Image caption Protesters clashed with police near the Houses of Parliament

Image copyright PA Image caption Six police horses were hurt during the fighting