The Unite for Europe march
comes just days before British Prime Minister Theresa May triggers Article 50
, which formally kicks off Brexit
negotiations between the UK and the European Union
Cheerful crowds gathered Saturday -- 60 years to the day since the signing of the Treaty of Rome
that established the EU's predecessor, the European Economic Community -- outside the Hilton hotel on Park Lane in the city's center.
Countless blue and gold European flags were hoisted high into the air, whipping in the wind, as people marched along Piccadilly and across Trafalgar Square, ending at Parliament Square, the scene of Wednesday's brutal rampage that left four dead
and more than 50 injured.
As crowds assembled in Parliament Square, people bowed their heads for a minute of silence in honor of those who were killed or injured in the horrific terror attack.
London's Metropolitan Police had announced via Twitter that more officers would be on duty for the march.
Former Deputy Prime Minister Nick Clegg and Alastair Campbell, a onetime communications adviser to former Prime Minister Tony Blair, were among those addressing the crowds.
"We have to persuade those who feel as we do to make themselves heard, not to be cowed by the bullying and intimidation of the Brextremist lie machine papers and not to be fooled by the sense of inevitability Mrs. May is trying to exploit," Campbell said.
"Of course, she is in a rush. Because the more time the public (has) to realize where this is heading, the more they will demand the right to have another say."
In the UK referendum last year, 51.9% said Britain should leave, while 48.1% chose to remain. Turnout was 71.8% -- well above the threshold required.
But many have argued since, including at Saturday's rally, that a second referendum is needed to allow British voters to decide on the terms and with full understanding of what an exit from the EU would entail.
Ian Clarke, a 57-year-old schoolteacher, told CNN he was at the protest because he "fundamentally disagrees with the Brexit vote."
"We think that the government is leading the country into a bad place and we need time to pause and reflect before things are decided," Clarke said. "I think that British people need the opportunity to think again and vote again."
Acknowledging that EU membership is not without difficulties, he was emphatic that Britain would be better off in the bloc than outside it.
Calls for unity
Miloslawa Kwiatek, a Polish doctoral student researching migration, came up from Bournemouth, a town on England's southern coast, to march Saturday with her friend Andrew Lee.
"We never wanted this, we never voted for this. I was disenfranchised as an EU citizen having lived here for so many years, and I think it's awful," Kwiadk said.
She said they were marching for unity, free movement within the EU and for the rights of those who want to make the most of what Europe has to offer.