LONDON — The political fallout from Britain’s stunning decision to leave the European Union intensified Sunday, with debate escalating inside the governing Conservative Party over choosing a successor to Prime Minister David Cameron and a coup attempt emerging against the leader of the opposition Labour Party.

The turmoil underscored the degree to which Thursday’s Brexit vote, described here as the most significant event in the post-war history of Britain, has left the country unsettled, virtually leaderless and in uncharted territory. The complex process of negotiating the terms of the separation from the European Union has collided with a leadership crisis triggered by a voter revolt against nearly the entire political establishment.

Overnight came news that Labour Party leader Jeremy Corbyn had fired one of the most senior members of his leadership team, Hilary Benn, the shadow foreign secretary. The sacking of Benn, whom Corbyn accused of plotting against him, led to the resignation of another member of Labour’s leadership team, with British news organizations reporting that additional resignations would be coming.

The Labour revolt reflects fears that the party could suffer what Benn called a “catastrophic” defeat if there is a general election this fall, after the Conservative Party chooses a new leader to succeed Cameron.

“He’s a good and decent man, but he is not a leader,” Benn said of Corbyn during an interview with the BBC’s Andres Marr.

Corbyn faces a vote of no confidence among his parliamentary peers this week, but has vowed to stand firm against efforts to oust him. He enjoys widespread support from Labour’s rank-and-file membership, setting up what could be a potentially disastrous civil war within the party just a year after he was chosen to lead it.

The fate of Corbyn is seen as secondary to the question of who will lead the Conservatives, as that person will immediately become prime minister. Boris Johnson, the former mayor of London and the leading voice in the campaign to exit the EU, is considered the favorite to take over from Cameron.

But the flamboyant Johnson is a magnet for controversy and there were reports Sunday of efforts by other Conservatives to deny him the post he has long been maneuvering to claim. One possible challenger is Theresa May, currently the home secretary and a quiet campaigner in favor of Britain remaining in the EU.

The political crisis came as European leaders continued to demand swift action by Britain to begin the process of leaving the EU and as the leader of the Scottish National Party pressed forward with another referendum to declare independence from the United Kingdom and seek a separate arrangement to stay in the EU.

Nicola Sturgeon, the Scottish party leader, reiterated her determination to move ahead with actions that could dismantle the U.K. Although Scotland’s voters rejected an independence referendum two years ago, circumstances have changed, Sturgeon said Sunday on the BBC.

Sturgeon also added a new element of uncertainty about the future course of Britain and the EU, raising the possibility of the Scottish parliament effectively trying to block Britain’s exit. In a later interview with the BBC, she said she would “of course” urge members of the Scottish parliament not to give consent to the withdrawal.

When Cameron on Friday announced his intention to resign, he said the formal process of withdrawal would not begin until a new prime minister is in place. But Saturday, foreign ministers from six original EU countries put pressure on Britain to begin that process immediately to avoid further instability resulting from a delay.

Cameron has sought a looser timetable to give Conservatives a chance to choose his successor, who would fashion a withdrawal deal over the course of two years. But technically, negotiations can’t start until Britain triggers the bloc’s Article 50 — the never-before-used mechanism to leave it.

Cameron has given no indication of pulling that lever fast. But patience was wearing thin for European officials stung by the Brexit vote.

“We start now,” French Foreign Minister Jean-Marc Ayrault told reporters in Berlin. “We must be clear. The British people have decided after an initiative that was taken by Mr. Cameron. That was, is his responsibility.”

The top diplomats from Germany, France, Italy, the Netherlands, Belgium and Luxembourg issued a joint statement Saturday that called for a start of exit talks “as soon as possible.” Their fast meeting underscored the continental effort taking shape to prevent further disintegration of European unity in the wake of the British decision.

German Foreign Minister Frank-Walter Steinmeier echoed calls for quick talks, warning of the risks of a drawn-out quagmire.

“We understand and respect the result and understand that Great Britain now concentrates on Great Britain,” Steinmeier said. But, he added, “this process should begin as soon as possible, so that we don’t end up in a long stalemate but are able to concentrate on the future of Europe and work on it.”

Two camps appeared to be lining up, however. Some European diplomats were eager to hold Britain’s feet to the fire for leaving — seeking a tough deal that would dissuade other member states from seeking similar referendums to leave the bloc.

But Steinmeier, as well as German Chancellor Angela Merkel, who will be instrumental in the exit talks, were, at least publicly, staking out a position that made Germany the adult in the room. Earlier Saturday, Steinmeier told Germany’s ZDF television that Europe should “not go looking for revenge.”

Speaking in Potsdam, Germany, Merkel sounded resolute but patient. “To be honest, it shouldn’t take forever,” she said of Britain’s exit. “But I wouldn’t fight over a short period of time.”

At the same time, deep intrigue reminiscent of ancient European power plays using knights and swords instead of the ballot box swirled over the future of Scotland.

Scottish voters, unlike the English or Welsh, voted Thursday to remain in the EU — and Scotland’s cabinet met in Edinburgh to consider its next steps. On Saturday, Sturgeon, Scotland’s first minister, confirmed that the party would begin efforts to seek a new vote on independence, a move that eventually could bring another wrenching change to an island nation that is now operating without a clear road map.

The “cabinet agreed that we will seek to enter into immediate discussions with the E.U. institutions and with other E.U. member states to explore all possible options to protect Scotland’s place in the E.U.,” she said.

During Scotland’s referendum for independence from Britain in 2014 — which failed — EU officials were circumspect, saying at the time that Scotland would have to get in line with the likes of Serbia and others who also wanted to join their club. If that position changes swiftly into an opportunistically strong pledge of membership now, it could drive an even deeper wedge between England and the continent.

Yet, already, some EU politicians have said that if Scotland extends a hand, the EU should take it — suggesting the potential tug of war ahead for Scottish loyalties between London and continental Europe.

“If Scotland wants to be a member of the European Union as an independent country, then they are welcome,” said Manfred Weber, the chair of the European Parliament’s center-right European People’s Party and an ally of Merkel. “But that’s up for them to decide. That shows that there are positive signals. Some people do want to be members of the European Union.”

Britain could undergo further dismantling if the nationalists in Northern Ireland, which also voted to remain in the EU, press ahead with their calls for a vote on Irish reunification. Signs at the main post office in Belfast warned all who entered that it had already run out of applications for passports from Ireland.

In the tumult, Jonathan Hill, Britain’s European commissioner, announced his resignation Saturday, citing his disappointment at the outcome of the referendum. His departure as Britain’s most senior official in Brussels was expected, but it nonetheless was another reminder of how the vote Thursday is quickly shrinking the country’s role in European affairs.

As Britons fretted over the consequences of their decision, a petition for a second referendum — considered a long shot for myriad reasons — garnered more than 2 million signatures.