Demonstrations are expected across Britain on Saturday against Prime Minister Boris Johnson's move to suspend parliament in the final weeks before Brexit.

The protests come ahead of an intense political week in which Johnson's opponents will seek to block the move in court and legislate against a no-deal departure from the European Union.

Organisers said they were hoping hundreds of thousands of people would take part in the demonstrations, organised under the slogan #StopTheCoup.

"Disruption is the only form of leverage protesters can rely on," said Michael Chessum, national organiser for the campaign group Another Europe is Possible.

"This process needs to force the government to change its course," he said.

The left-wing group Momentum, closely allied with the main opposition Labour Party, is calling on its supporters to "occupy bridges and blockade roads".

Saturday's main demonstration is expected outside Johnson's Downing Street office in London from around 1100 GMT and there will be more in around 30 other towns and cities.

Johnson warns MPs

Johnson, who only came to power a month ago following a Conservative Party leadership election, has promised to lead Britain out of the EU on October 31 with or without an agreement with Brussels.

He has said he is ready to strike a deal as long as provisions for Britain to stay in the customs union even after Brexit are cut from an existing deal struck by his predecessor Theresa May.

But EU leaders have said they are still awaiting concrete proposals from London.

Johnson's Brexit adviser David Frost is expected back in Brussels for talks next week.

Opposition MPs and some lawmakers from Johnson's own Conservatives want him to delay Brexit beyond October 31 if he fails to strike an agreement with the EU.

The government meanwhile is ramping up preparations in case of no-deal.

Johnson on Friday cautioned MPs against trying to hamper his plans, saying a decision to delay Brexit again would do "lasting damage" to public trust in politics.

He said the opposition's efforts could in fact help lead to a no-deal Brexit as EU counterparts would be less likely to offer a compromise if they believed Brexit could be stopped.

1.6 million signatures

Queen Elizabeth II gave her approval to Johnson's decision to suspend parliament for several weeks on Wednesday, sparking widespread outrage, legal challenges and promises of resistance from parliamentarians.

The move was widely seen as a way of limiting the time Johnson's opponents have to organise against him.

The main opposition Labour Party has said it is also mulling a no-confidence vote in Johnson's Conservative government, which commands a fragile 320 to 319 majority.

An online petition calling for the government to reverse its suspension has garnered 1.6 million signatures.

In the courts, a Scottish judge is expected to hear a legal challenge against the suspension on Tuesday -- the same day MPs return from their summer break for their shortened parliamentary session.

There will be a separate court hearing on Thursday for another challenge that is being supported by John Major, a former Conservative prime minister and staunch opponent of Brexit.