The protesters waved the blue and gold flag of the European Union and held up "Bollocks to Brexit" banners under sunny skies to call for another referendum on the eventual deal on how Britain will leave the world's biggest trading bloc.
Council leader Keith House added: "All evidence is that Brexit will cost jobs in all our communities, push up prices and hit hardest those people that have had the toughest time with forced austerity".
With a bill enacted late a year ago, Parliament gave itself the right to hold a final vote on any Brexit deal before May can complete it.
Khan said Saturday's protest was a "march for the future" for young Britons, including those who were too young to vote in Britain's 2016 Brexit referendum, when those who favored leaving the European Union won narrowly with 52 percent.
Ms Soubry said: "We are winning the argument, most importantly against those who voted leave". She said recently SNP MPs would back a new Brexit referendum.
Mayor of London Sadiq Khan was among those who spoke in the square, along with representatives from the main political parties. Thousands of people are expected to join a People's Vote march in central London on Saturday.
The group met outside the Hilton hotel on Park Lane, but Helen said they did not move until 1.50pm and failed to get into Parliament Square for the afternoon's rally as it was so busy.
Thousands of people filled the streets, some were singing, others were playing instruments, while many plastic whistles blared out. This may be sufficient for her opponents (not just the general opposition, but also opponents within the ruling Conservative Party) to start seeking new parliamentary elections and, as a effect, the new Prime Minister of the United Kingdom.
In a 2012 speech, Davis said: "If a democracy can not change its mind, it ceases to be a democracy" - but appeared to change his own mind previous year when he rejected calls to put a clause into the Brexit bill allowing the government to reverse Brexit. That's why the protesters want to make sure they get a second chance to vote on Brexit once they know exactly what the deal will be. Remainers know that time is running out, with the United Kingdom [1.58] to leave the European Union by March 29 next year, and they know they need to make their point louder and clearer than ever. "To demonstrate against this threatens our democratic process, and while I am glad the march went off peacefully, we should be very wary of where the anti-Brexit lobby are taking us". Marchers held up banners suggesting they came from all corners of the country, while others were Spanish, French and Italian.
The CBI also said that 80 per cent of firms say Brexit has already had a negative impact on their investment decisions.
"It's now or never really, we're getting to crunch time".
According to AFP, polls show support for a second referendum evenly split.