As MPs on both sides of the Brexit divide battled it out in the Commons ahead of the vote on Theresa May's deal, an even noisier and more passionate debate was going on across the road.
With fancy dress, lurid floats and colourful banners, there was something approaching a carnival atmosphere, as the usual ranks of pro and anti-Brexit demonstrators outside Parliament were swollen by people who had travelled to London for the big vote.
The two sides mostly kept a respectful distance from each other, as a low-key police presence tried to keep a right of way open for pedestrians.
The protest was at times loud - with a group of pro-Brexit demonstrators ringing a "liberty bell" and beating a bass drum, with tensions escalating as the MPs' vote approached.
Everyone - with the notable exception of a man wielding a "leave, then negotiate" placard - appeared to be united in their disdain for Theresa May's Brexit agreement.
"It's a rubbish deal," said retired cab driver Colin Grostate, a card-carrying Conservative Party member, and prospective council candidate.
"Everybody in the country knows, except Theresa May. The lights are on but nobody's home.
"The woman's a nutcase. She's just in denial."
Mr Grostate, like some of the other pro-Brexit demonstrators, was wearing a yellow vest, in solidarity, he said, with "working class" protesters in France. He was angry about what he saw as a betrayal of the result of the 2016 referendum by MPs.
"We voted for freedom, not for money. We didn't say we wanted to be richer or poorer. We voted for freedom."
Mr Grostate, who lives in Ramsgate, has booked himself into a London hotel for four days to protest outside Parliament.
"I am willing to die for the vote. If they don't leave on the 30 March, there's thousands like me up and down the country that is willing to stand their ground.
"We lost 7 million British, Commonwealth and Americans fighting for this country to be free. We are not handing it over to the Europeans, that's for sure."
Peter Hayden, a Labour Party member, was equally angry, albeit in a more understated way, with his party leader.
He says he joined Labour because of Jeremy Corbyn's socialist policies - "I was a Corbynite" - but has become disillusioned with Mr Corbyn's refusal to get behind calls for another EU referendum, which is what, he says, the vast majority of Labour members want and what they voted for at their party conference.
"He needs to come off the fence because otherwise the initiative will be taken by the Tories, so Labour will be sidelined on this whole issue."
Like Mr Grostate, the retired author, from Warwickshire, is in London for the week to make his voice heard.
"This is fanatics' alley," he says, surveying the protests going on around him.
"Everyone who is here, I suppose you could say including me, they are coming out because they feel fanatical, so you can't gauge opinion. The polls say there is a drift towards Remain and I hope they are right."
Alena Useinviz is part of a group of EU citizens, In Limbo, protesting against what they believe is the reduction in rights that will flow from Mrs May's deal.
"I have lived here for 10 years legally. I have always paid tax. I have worked all my life. I now have to apply for the right to remain here. And I do not like that."
She added: "As a British citizen with a German passport I think it's a bad deal. I would like to see it returned to the people. I would also like it to be inclusive of the European citizens here as well as the British citizens in Europe."
Victor Zanchi had dressed up as a "plague doctor" because, "Brexit is taking us back to the Middle Ages".
The Italian citizen, who says he could not vote in the 2016 referendum, has since taken British citizenship, and is down for the day from York to make his voice heard, ahead of the vote.
"Ideally I would like Brexit completely stopped but I understand why that is not a straightforward thing to happen and a second referendum is the most reasonable way towards that."
Mark Day, a planning officer from Horsham, West Sussex, who was draped in an EU flag, said he would never have expected to take to the streets in protest at Brexit or join a political party - the Liberal Democrats - before the 2016 referendum.
"I wasn't really active and I feel ashamed about that in many ways. Because basically I thought the country's not that stupid," he said.
He now feels he has to act, he says, because Brexit is "the most disastrous thing that can ever happen to us in peacetime".
A contingent from UKIP was also out in force, waving their Leave Means Leave placards at motorists, eliciting the occasional supportive honk of the horn.
One of them shook his head, as a bright yellow anti-Brexit bus cruised past for the umpteenth time. "It's not very civil is it?" he said, gazing at the rude slogan emblazoned across it.
"We are trying to say don't pass this withdrawal bill," said Marietta King, a member of UKIP's national executive committee, down for the day from the Midlands.
She said she voted to stay in the EEC, as it was then called, in the 1975 referendum, but that was when it was purely a trading arrangement, she says, before it "took control" of so many laws.
"I would rather go with no deal, save the money, put it to better use here, and then teach our own MPs to rule. They haven't done it in a long time."