Pro-EU supporter Sir Bob Geldof, actor Sir Patrick Stewart and Tony Blair’s former spin-doctor, Alastair Campbell, are among those who have backed the event, which terminates Parliament Square, and where Sir Bob will address the crowd.

Earlier crowds gathered at Hyde Park corner to hear speakers including Lib Dem leader Vince Cable, who described the event as " a really great demonstration of our support for Britain's participation in Europe.

"We've got to absolutely stop the Brexit which this rather incompetent, disfunctional and disunited government are trying to force upon us," he continued.