Thousands of masked protesters have descended on central London for the annual Million Mask March.
Wearing trademark Guy Fawkes masks, supporters of the mysterious hacking collective Anonymous descended on Trafalgar Square as scores of police officers maintained a tight perimeter.
The protest is one of many similar marches held worldwide on November 5. Its agenda is broadly anti-capitalism and pro-civil liberties.
Ugly scenes marred last year's march, with missiles and fireworks thrown at police, leaving four officers injured.
The Metropolitan Police imposed restrictions on this year's event amid concerns over any repeat of the disorder.
Police have imposed conditions on the march, limiting it to a three-hour period between 6pm and 9pm on a prescribed route between Trafalgar Square and Whitehall.
Any static protest must only take place in Trafalgar Square, Richmond Terrace and Parliament Square, and the Met has warned that anyone breaching the conditions could be arrested for public order offences.
The protest began peacefully, with several participants climbing the base of Nelson's column and chanting the slogan "one solution, revolution".
But there were angry scenes as the procession continued, with police forming a ring of steel outside Parliament.
Officers stood firm, telling the protesters to stay within the boundaries.
Several supporters, masked and otherwise, argued they should be free to pass on public roads.
While most opted for a Guy Fawkes mask, the crowd was also dotted with people dressed in Darth Vader helmets, while others were wrapped in scarves.
Plumes of smoke wafted into the cold night air as the protesters moved past Big Ben, with police marching behind.
Later, some participants paused to read a message projected in green letters onto a building lining Parliament Square.
It read: "Please observe Public Order Act restrictions. Failure to comply may result in arrest and prosecution. Officers may require you to remove facial covering. Failure to comply is an offence."
It came after members of the crowd ignited fireworks and flares in front of Westminster Abbey.
The protest in Parliament Square dwindled by several hundred people at around 7.30pm as one man was seen being led away by officers.
The Met said there had been 10 arrests as of 7pm, including one for possession of an offensive weapon, six for drugs and two for the non-removal of a face mask.