Protesters wielding pro-democracy placards and EU flags rallied on Saturday in dozens of British cities against Prime Minister Boris Johnson's controversial move to suspend parliament just weeks before Brexit.

In the biggest demonstration, thousands of whistle-blowing, drum-banging people gathered raucously outside the gates of Downing Street in London chanting "Boris Johnson shame on you!" 

"I'm absolutely disgusted by what's happening here," said attendee Maya Dunn, 66, a Dutch citizen living in Britain, who accused Johnson of "riding roughshod over everybody."

"You just can't trust him," she said.

The demonstrations come ahead of an intense political week in which Johnson's opponents will go to court to block his move to suspend parliament from mid-September and legislate against leaving the European Union without an agreement.

Johnson, who only came to power in July following a Conservative Party leadership election, hit out at the prospect in a newspaper interview published Sunday.

"What on earth are we achieving by this?" he told the Sunday Times, noting Brexit had already been delayed twice this year and warning rebel Tory MPs risked toppling the ruling Conservatives. 

"What we need to do is get a deal done, or if we can't get a deal done then get out of the EU on October the 31st, come what may. 

"And that's what we're going to do," he added.

However, the EU's chief Brexit negotiator said Brussels would not change the divorce deal struck with his predecessor Theresa May.

"I am not optimistic about avoiding a 'no deal' scenario," Michel Barnier wrote in the Sunday Telegraph newspaper.

Johnson's parliament suspension was widely seen as a way of limiting the time his opponents have to organize against him.

In London, participants heard speeches from opposition politicians on a stage erected on Whitehall before marching through Westminster. Some held hand-written signs reading "Defend democracy: resist the parliament shutdown" and "Wake up UK! Or welcome to Germany 1933."

Organizers using the slogan #StopTheCoup claimed as many as 100,000 people turned out in London.