Hundreds of thousands of marchers filled London streets Saturday to demand a second Brexit referendum as pressure builds on Prime Minister Theresa May to resign.

The massive demonstration--one of the city's largest protest gatherings in years-- took place with May unable to convince Parliament on a plan for the U.K. to leave the European Union--despite two attempts in the House of Commons.

Several British newspapers, including the London Times and The Daily Telegraph, have reported that pressure is growing on May to resign.

The "Put It To The People" demonstrators planned a rally in front of Parliament after marching from Park Lane to Parliament Square.

THERESA MAY'S BREXIT DEAL FACES NEW VOTE IN BRITISH PARLIAMENT: WHAT TO KNOW

“I would feel differently if this was a well-managed process and the government was taking sensible decisions. But it is complete chaos,” demonstrator Gareth Rae, 59, told Reuters. “The country will be divided whatever happens and it is worse to be divided on a lie."

A 2016 referendum to leave the EU passed by 1.3 million votes.

London Mayor Sadiq Khan tweeted a video of himself with demonstrators holding up a 'Put it to the People' banner at the front of the march as it began.

Joining him was Liberal Democrat leader Vince Cable, who tweeted that there was a "huge turnout of people here from all walks of life."

The embattled May wrote to lawmakers Friday night saying she would only bring the European Union withdrawal plan back to Parliament if there seems to be enough backing for it to pass.

“If it appears that there is not sufficient support to bring the deal back next week, or the House rejects it again, we can ask for another extension before 12 April, but that will involve holding European Parliament elections,” she said.

EU LEADERS AGREE TO SHORT-TERM BREXIT DELAY, GRANTING PM THERESA MAY A LIFELINE

The Associated Press interviewed 63-year-old Edmund Sides, who spent the last three weeks walking from Wales to London in order to take part in the demonstration.

He expressed worry about the vicious tone that arguments have started to take and worries about national cohesion.

CLICK HERE TO GET THE FOX NEWS APP

“People fear the atmosphere is very dangerous in this country,” he said.

The Associated Press contributed to this report.