Thousands of anti-Brexit demo in London

Central London was brought to a standstill on Saturday as tens of thousands of people gathered to protest against Britain's departure from the European Union (EU).

From the London's Park Lane to the Houses of Parliament, the crowds, many carrying pro-EU banners, marched more than 3 kilometers to demonstrate their call for Britain to remain within the EU.

The protest came on the day when leaders of 27 EU member states gathered in Rome to celebrate the 60th anniversary of the Treaty of Rome which paved the way for the EU.

It also came just days before British Prime Minister Theresa May finally triggers Article 50, the mechanism laid down by the EU for a member state wishing to leave. May has announced she will send her letter next Wednesday to European Commission President Jean-Claude Juncker.

March organisers, Unite for Europe, held talks with the Metropolitan Police in London and city officials following this week's attack on Westminster. The authorities agreed the march could continue as planned.

A number of leading pro-EU politicians addressed the crowds during the demonstration. The event, accompanied by a band, went quiet during a minute's silence in memory of those killed and injured in this week's attack.

One of the speakers, MP Nick Clegg who was deputy prime minister in David Cameron's coalition government, told the crowd: "I was profoundly saddened by the outcome of the referendum, but that sadness has given way to a perpetual sense of anger about the choices that Theresa May and her government have taken since."

He accused May of threatening to turn Britain into "a bargain basement cowboy economy."

London MP David Lammy said there are a lot of people against Brexit in Britain, adding that in democracies people are always allowed to change their minds.

Many EU nationals living in London joined the march, while some demonstrators waved EU flags to show their support.

A spokesman for Unite for Europe said: "We want to remain in the European Union. We value the peace, friendships, relationships, rights, business and academic collaborations, economic prosperity and outward-looking attitude that EU membership delivers.

"We are the 48 percent, who voted against Brexit and those who were not allowed to vote against it -- the young and the EU nationals living, working and paying taxes in the UK. We are outraged by the government's current direction in dealing with the result of the referendum."

The march also came as the MP Douglas Carswell, the only politician in the House of Commons from the pro-leave UKIP party, announced his resignation from UKIP. He will continue to serve as an MP as an independent member.

Explaining his reason for leaving, Carswell said following the referendum result by Britain to leave the EU, the job of UKIP had been done. Carswell had fallen out with former UKIP leader Nigel Farage who said on social media that Carswell had jumped before he could be pushed.