Marchers set off in central London with banners proclaiming "the best deal is no Brexit" and "we demand a People's Vote" in what organizers said could be the biggest anti-Brexit protest yet.
After three years of tortuous debate, it is still uncertain how, when or even if Brexit will happen as May tries to plot a way out of the gravest political crisis in at least a generation. May hinted Friday that she might not bring her twice-defeated EU divorce deal back to parliament next week, leaving her Brexit strategy in meltdown.
The Times and The Daily Telegraph reported that pressure was growing on May to resign.
"I would feel differently if this was a well-managed process and the government was taking sensible decisions. But it is complete chaos," Gareth Rae, 59, who traveled from Bristol to attend the demonstration, told Reuters. "The country will be divided whatever happens and it is worse to be divided on a lie."
While the country and its politicians are divided over Brexit, most agree it is the most important strategic decision the United Kingdom has faced since World War Two. Pro-EU protesters gathered for a "Put it to the people march" at Marble Arch on the edge of Hyde Park around midday, before marching past the prime minister's office in Downing Street to finish outside Parliament.
While there was no official estimate of the numbers, campaign organizers said hundreds of thousands of people were in the crowd as it began to march. Organizers were confident that the size of the crowd would exceed a similar rally held in October, when supporters said about 700,000 people turned up.
"We have come here today because we feel like our future has been stolen from us. It is our generation that is going to have to live with the consequences of this disaster," Phoebe Poole, 18, who wasn’t old enough to vote in the 2016 referendum, told Reuters.
"It is going to make it harder to get a job. You are already seeing a lot of large companies leaving. I am worried about the future," she added.
Two hundred buses from around Britain were booked to take people to London for the march. A petition to cancel Brexit altogether gained 4 million signatures in just 3 days after May told the public "I am on your side" over Brexit and urged lawmakers to get behind her deal.
In solidarity, around 100 Britons living in Madrid, Spain, also held an anti-Brexit rally against Britain leaving the European Union.
The protesters chanted slogans against Brexit and many also held up homemade signs, Public’s Radio reported.
In the June 23, 2016 referendum, 17.4 million voters, or 52 percent, backed Brexit while 16.1 million, or 48 percent, backed staying in the bloc.
Ever since, however, opponents of Brexit have been exploring ways to hold another referendum.
May has repeatedly ruled out holding another Brexit referendum, saying it would deepen divisions and undermine support for democracy. Brexit supporters say a second referendum would trigger a major constitutional crisis.
We already put it to the people. And the people roared," pro-leave group Change Britain said in a tweet.
Supporters of Brexit say that while the divorce might bring some short-term instability, in the longer term Britain would thrive if cut free from what they cast as a doomed experiment in German-dominated unity that is falling far behind other major powers.
Some opinion polls have shown a slight shift in favor of remaining in the European Union, but there has yet to be a decisive change in attitudes.
Many voters in Britain say they have become increasingly bored by Brexit and May said Wednesday that they want this stage of the Brexit process to be "over and done with."
But protesters disagreed with May's claim that she is on the side of the British public, with one placard reading: "You do not speak for us Theresa."