Brexit 'People's Vote' march - live updates
Former deputy prime minister Michael Heseltine criticized Theresa May in a speech in Parliament Square.
“The Prime Minister herself bears a heavy personal responsibility for our current crisis,” he said, adding that her criticism of MPs last Wednesday was an "affront to parliamentary democracy."
Generals who lose wars blame their troops, managers who break their companies blame the workers, said Heseltine.
"Now we can add: Prime ministers who lose elections blame their MPs," he said, before calling for a second referendum.
"One way or another, you the people must decide. You the people must be free to vote to remain."
Private equity director Jonty Graham, 35, and PR executive Charlotte Graham, 32, are attending the protest with their daughters Poppy, 5 and Tilly, 4.
"We are sleepwalking towards disaster," Jonty Graham told CNN.
"I have lots of friends who I play with who are French and Spanish and I like playing with them," said Tilly.
The Independent Group, a cross-party bloc of rebel anti-Brexit MPs, has posted a time-lapse video showing the crowd in Parliament Square.
MP Anna Soubry, who recently left the Conservative party to join a breakaway cross-party bloc known as the Independent Group, spoke to crowds in Parliament Square.
In a video posted on Twitter by Independent Group MP Chris Leslie, Soubry said former Conservative colleagues have told her in private that they know a so-called "people's vote" is the only solution.
"Put your country first," she said, urging MPs to support a second referendum.
London Mayor Sadiq Khan addressed the crowds in Parliament Square this afternoon, and also called for a People's Vote on Twitter.
Take a look at a few of these protest signs for a bit of light relief.
Passionate Remainer Charles, 63, a retired teacher from Ripon in North Yorkshire, told CNN that the EU was not perfect but it has contributed to peace in the last 70 years.
"The country had the chance to vote on a principle but now we know a bit more of what the reality would be and people need the chance to say if they want that reality," he added.
If people say they want Brexit now -- knowing what it involves -- I would respect that, Charles added.