LONDON (AFP) - Tens of thousands of pro-EU protesters took to London's streets on Saturday (March 25), in defiance of the terror threat, to mark the bloc's 60th anniversary just days before Brexit begins.

Organisers said around 80,000 people had joined the march calling for Britain to stay in the European Union, even as Prime Minister Theresa May prepares to start the withdrawal process on Wednesday.

A sea of blue EU flags stretched down Piccadilly and past Trafalgar Square, interspersed by signs saying "I am European" and "I'm 15 - I want my future back!"

The crowd fell silent as it filed into Parliament Square, the scene of terror this week when a homegrown killer drove a car through crowds of people before crashing into parliament and stabbing a policeman.

"Terrorism won't divide us - Brexit will," said one banner held aloft, while another said: "Stop sleepwalking, stop this madness." There were calls to cancel the march after Wednesday's attack, which left four people and the perpetrator dead, but organisers said: "We will not be intimidated."

Police said that "an appropriate policing plan is in place" but a reporter said security was discreet.

In a referendum on June 23, Britons voted by 52 per cent to end their four-decade membership of the EU.

But 48 percent voted to stay - and are unhappy with May's plans to leave the EU's single market in order to cut immigration, and her refusal to guarantee the rights of three million Europeans living in Britain.

"I was told I could settle down, marry a Brit and make my life here," said Joan Pons, a Spanish nurse who has lived in Britain for 17 years. "Yet today I am told I'm a foreigner and should go back where I come from."

On Wednesday, the prime minister will trigger Article 50 of the EU's Lisbon Treaty, starting a two-year countdown to Britain's exit.

She declined to attend celebrations in Rome on Saturday marking the EU's creation, when six founding states signed the Treaty of Rome on March 25, 1957.