London (AFP) - Prime Minister Boris Johnson's suspension of parliament just weeks before Britain's EU departure date faced legal challenges on Thursday amid a furious outcry from pro-Europeans and MPs opposed to a no-deal Brexit.

Johnson announced the surprise decision Wednesday to dismiss parliament next month for nearly five weeks, claiming his new government needed a fresh start in order to pursue a "bold and ambitious" post-Brexit domestic agenda.

But the move ahead of the October 31 Brexit date sent shockwaves through British politics.

Johnson's opponents labelled the suspension of parliament a "coup" and a "constitutional outrage".

The prime minister insists Britain must leave the European Union on time, with or without a divorce deal -- and the move limits his opponents' room for manoeuvre.

It prompted immediate court bids in London, Edinburgh and Belfast to halt the process.

An interim ruling is expected in the Edinburgh case on Friday.

Major protests were planned for Saturday as politicians scrambled for ways to counter Johnson.

The leaders of the six opposition parties in parliament condemned Johnson's "undemocratic" actions in a joint statement, saying they believed a majority of MPs were against the move.

"We demand that the prime minister reverses this decision immediately or allows MPs to vote on whether there should be one," they said of the suspension.

"The prime minister is shutting down parliament with the sole aim of stopping MPs from avoiding a no-deal Brexit.

"Voters are being deprived of the opportunity to have their representatives hold the government to account."

Labour opposition leader Jeremy Corbyn said he would try to start a parliamentary process that would allow Johnson's opponents to legislate to prevent Britain leaving the EU without a deal and to stop the suspension of business.

Corbyn is also mulling a no-confidence vote in Johnson's Conservative government, which commands a fragile 320 to 319 majority.

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Former Conservative finance minister Ken Clarke, parliament's longest-serving MP, has been touted as a unity figure to lead a temporary alternative government.

If it could lead to a "sensible outcome of the present crisis, then yes," Clarke said of the idea.

As for voting to bring down his own government, Clarke told ITV television: "If it's the only way of stopping us plunging into the disaster of a no-deal Brexit, then yes."

- Court cases -

Gina Miller, a businesswoman and leading anti-Brexit campaigner, has applied for an urgent judicial review in London challenging "the effect and the intention" of the suspension.

"We think that this request is illegal," said Miller who in 2017 successfully won MPs the right to vote on formally starting to leave the EU in a court challenge.

In Scotland's highest civil court, 75 parliamentarians are seeking an interim interdict that would stop Johnson suspending parliament pending a final decision on the case.

Defence Secretary Ben Wallace was caught on an open microphone at a Helsinki summit saying parliament was unable to agree what it wanted.

"So eventually any leader has to try. I don't know what the outcome...," he said, before laughing.

A Downing Street source said Wallace "misspoke". Johnson and the government insist the suspension is completely legal and simply a routine parliamentary procedure without a political objective.

Scottish parliamentarian Joanna Cherry, who is bringing the Edinburgh case, said the minister's comment "backs up what we contend -- that prorogation of parliament is for an improper purpose and therefore unlawful".

Meanwhile, campaigner Raymond McCord launched a legal bid in Belfast to block Johnson's move, with a hearing set for Friday.

"He's obviously trying to circumvent parliament and we say in the context of Northern Ireland that is unconstitutional," McCord's lawyer Ciaran O'Hare told AFP.

- 1.5m sign petition -

An online petition seeking to block the decision unless Brexit is delayed or cancelled had garnered more than 1.5 million signatures on Thursday.

Thousands of people protested Wednesday in London, Manchester, Edinburgh and other cities.

General protests are planned outside parliament and around Britain on Saturday.

Labour's leftist faction Momentum urged members to "occupy bridges and blockade roads".

Meanwhile, leftists will rally outside parliament on Tuesday demanding an immediate general election.

After its longest session in nearly 400 years, parliament will now close in mid-September and reopen on October 14 -- just over two weeks before Brexit.

The House of Commons typically goes into recess around the annual party conference season, which kicks off on September 14 and ends on October 2, but critics slam this lengthier break.

The pound was stable on Thursday after sliding on news of the suspension.