Thousands of protesters flooded central London on Saturday for the annual "Million Mask March" under tighter restrictions imposed this year after violent confrontations with police in 2015.

As of 10:30 p.m. local time, police had reported 33 arrests in connection with the march, The Telegraph reported.

The protesters began their march at Trafalgar Square, heading toward Westminster chanting, "Whose street? Our Streets!"

Marchers, many wearing Guy Fawkes masks, were under a strict three-hour time limit this year as a result of battles between police and protesters last year that led to more than 50 arrests.

According to The Telegraph, Scotland Yard said the 33 arrests at this year's march included three for possession of an offensive weapon, 14 for drugs and two for the non-removal of a face mask. Fireworks were set off in the crowd near the Houses of Parliament and bottles were thrown.

This year, scores of police maintained a tight perimeter around the marchers, who were restricted to an area from Trafalgar Square to Richmond Terrace and Parliament Square.

The worldwide event, which falls on Guy Fawkes Night, is organized by the hacktivist collective Anonymous UK. Protests were also planned for New York, Los Angeles, Edinburgh and Brussels.

Guy Fawkes Night, also known as Bonfire Night, falls on Nov. 5 and marks the foiling of a plot by a group of Catholics to kill King James I, a Protestant, in 1605. The plan was to blow up the Houses of Parliament during its state opening.

Fawkes, an explosives expert, was found beneath the building with 36 barrels of gunpowder and was sent to the Tower of London where he was tortured and later died. An effigy representing him is burned on bonfires around the country on the night of Nov. 5 to mark the events.

The "mask" refers to the Fawkes mask that many protesters wear that features a goateed figure with a moustache, large eyebrows and an upturned smile.

On the eve of the event in London, Metropolitan police commander B.J. Harrington made it clear authorities would not allow the march to get out of hand this year.

"We saw participants causing criminal damage to public property, smashing the windows of businesses and attacking police officers all whilst harassing and intimidating families as they visited theaters, dined out or shopped in the West End," Harrington said of last year's march.

"As we look ahead to this weekend, my message is simple: If you want to protest peacefully, that is your right and we want to work with you," he added. "If you commit criminal acts — that is not peaceful protest — and you are liable to be arrested."

In a sign of the tenor of the planned protest, organizers warned on their Facebook page for the event that the police "are not your friends."

"Keep an eye out for your comrades and police tactics that will limit movement, the hive mind should stay vigilant," they said.

The protest group, in a rallying cry, pointed to what it said was government overreach and the violation of individual rights.

"We have seen the pushes to make the Internet yet another part of the surveillance state, we have seen the government's disregard for migrants, for the poor, the elderly and the disabled, we have seen the capital, profit and greed of the few put before the well-being of the many and we say enough is enough," the statement from Anonymous UK added, calling on all people who "want to see a positive change in the world, to join us."