By Jack Guy, CNN
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.
Anti-Brexit protesters have flooded towards London from across the country, bringing multiple generations of families together.
Some are carrying babies, others are celebrating their birthdays with family and friends, and relatives have met up together in the capital to protest.
Boscy, Robyn and Muffie have traveled from Brighton, Woking and Bath respectively. The niece, daughter and mother are on their third march in two years since Article 50 was triggered.
“We must revoke," said Muffie. "We owe it to the people."
Demonstrators are congregating in Parliament Square ahead of planned appearances from a number of anti-Brexit figures.
Saturday's route for the "Put It To The People" march saw protesters start at Park Lane before heading towards Parliament Square in central London.
London mayor Sadiq Khan, Labour's deputy leader Tom Watson, Scottish First Minister Nicola Sturgeon, and former deputy prime minister Michael Heseltine are all set to address protesters.
A million people are marching in London Saturday, according to a spokesperson for the organizers.
CNN has not been able to verify this figure and London's Metropolitan Police are not commenting on crowd estimates.
But if organizers are correct, that would mean Saturday's demonstrations have drawn significantly more than the 700,000 that turned out for the People's Vote march back in October.
One protester, Rebecca from Glastonbury, has brought her 15-month-old daughter Seren along for her first march.
Protesters have traveled from far and wide to protest in the British capital.
Rebekah, Colin and their sons Sam and John Paul came from Nottingham, in central England.
“Put it to the people again,” said Rebekah, who is advocating for a second referendum, or People’s Vote.
“We want to make sure our sons have the same chances we had,” said Colin.
Another protester, Roger from London, told CNN he was worried about the economic impact of Brexit.
"Brexit is self-evidently bad for the economy, it's bad for my kids' future," he said.
While anti-Brexit crowds gather in London, a rival set of marchers who support Brexit continue to move closer to the British capital.
The "March to Leave" set off from the city of Sunderland, in northeast England, last Saturday and hopes to reach London by March 29, the original Brexit day, reports the Press Association.
Staunch Brexit supporter Nigel Farage made a speech to marchers this morning near Nottingham.
"What has happened this week is not only a national humiliation but it is an outright betrayal because Mrs. May now tells us we're not leaving next Friday despite telling us over 100 times that we would be," he told PA.