London (AP) -- Britain's shocking decision to remove itself from the European Union brought more political turmoil Sunday as Scotland's leader threatened to block the move and the opposition Labour Party's leader faced a coup attempt from his own legislators.
The sense of unease spread as European leaders stepped up the pressure on Britain to begin its complex exit from the 28-nation EU immediately, rather than wait several months as British Prime Minister David Cameron prefers.
The vote to leave sent the pound and global stock markets plunging. Britain's Treasury said finance minister George Osborne would make an early morning statement Monday "to provide reassurance about financial and economic stability" before the London Stock Exchange reopens.
The leaders of the successful campaign to leave the EU stayed largely out of the public eye, as opponents accused them of lacking a plan to calm the crisis the result has triggered. In his first statement since Friday morning, "leave" leader and former London Mayor Boris Johnson used his column in the Daily Telegraph newspaper to urge unity and say "the negative consequences (of the vote) are being wildly overdone."
He said Britain would forge "a new and better relationship with the EU — based on free trade and partnership, rather than a federal system."
The vote, however, risks causing a political schism in the United Kingdom. Scottish First Minister Nicola Sturgeon said she would "consider" advising the Scottish Parliament to try to use its power to prevent Britain from actually leaving the EU. She said Scottish lawmakers might be able to derail the move by withholding "legislative consent" for a British exit, or Brexit.
"If the Scottish Parliament was judging this on the basis of what's right for Scotland, then the option of saying 'We're not going to vote for something that is against Scotland's interests,' of course, that is on the table," she said of the possibility of withholding consent.
Sturgeon said she believes Scotland's approval is required for the move but conceded the British government would likely take "a very different view."
Thursday's U.K.-wide vote to leave the EU was very unpopular in Scotland, where 62 percent cast ballots to stay, and Sturgeon says she is studying ways to keep Scotland part of the EU bloc.
The Scottish question looms large because Sturgeon also has said another referendum on Scottish independence from Britain is "highly likely" as a result of Britain's EU vote. A Scottish referendum in 2014 ended with voters deciding to remain in Britain, but analysts believe Britain's withdrawal from the EU may strengthen the independence movement.
In Northern Ireland, which also is part of the U.K., Deputy First Minister Martin McGuinness said his priority is forging "special arrangements" to enable Northern Ireland to maintain its EU ties. Some Brexit opponents have also talked of trying to use Northern Ireland's Assembly to try to block Britain's departure.
Northern Ireland voters also expressed a preference for keeping Britain in the EU. The unhappiness with the results in both Scotland and Northern Ireland is adding to the sense that the Brexit vote may over time lead to the breakup of the United Kingdom.
Northern Ireland Secretary Theresa Villiers, Cameron's lead official in Belfast, played down the suggestion that the Scottish Parliament or the Northern Ireland Assembly had the standing to prevent a British departure from the EU.
She said decision-making power resides solely in the British Parliament, which is expected to abide by the results of the referendum, which showed 52 percent of British voters wanted out.
"In the weeks and months ahead, we will be working with both the Scottish government and the Northern Ireland executive on all these matters," she told BBC. "But ultimately it is (the British) Parliament's decision."
Adam Tomkins, a law professor and member of the Scottish Parliament, agreed with this assessment. The Conservative Party legislator tweeted that it was "nonsense" to suggest the Scottish party could block a British departure simply by withholding consent.
The vote is already cutting short Cameron's career. He said after the results that he will resign as prime minister when the Conservative Party chooses a new leader, who will be charged with implementing the separation from the EU.
The new party leader, who will become prime minister, is expected to be in place by October. At that point, he or she may choose to call a quick election to solidify a mandate — and the prospect of an election in the near future may have spurred a revolt Sunday against Labour leader Jeremy Corbyn that has been simmering for months.
Corbyn, a longtime critic of the EU who was criticized by many for doing a weak job presenting the party's position favoring membership, for the first time faces an open rebellion from senior members of his "shadow cabinet" — the opposition party's mirror government of senior lawmakers.
Eleven "shadow cabinet"members resigned Sunday after Corbyn fired shadow Foreign Secretary Hilary Benn overnight for reportedly plotting a rebellion against him. The dissidents want Corbyn, who represents the far-left wing of the party, ousted before the next general election because many believe he cannot win.
In her resignation letter, shadow Health Secretary Heidi Alexander bluntly told Corbyn he had to go.
"I do not believe you have the capacity to shape the answers our country is demanding and I believe that if we are to form the next government, a change of leadership is essential," she wrote.
In a statement released late Sunday, Corbyn said he would not resign and would run in any new leadership contest. Senior allies said he still has strong support among the party's rank-and-file members, who chose him as leader last year.
"I regret there have been resignations today from my shadow cabinet," Corbyn said. "But I am not going to betray the trust of those who voted for me — or the millions of supporters across the country who need Labour to represent them."
Concerns about last week's EU referendum ranged far beyond U.K. politics.
In Rome, U.S. Secretary of State John Kerry urged Britain and the EU to manage their divorce responsibly for the sake of global markets and citizens. On Monday, he will be the first senior U.S. official to visit London and Brussels since the referendum, and he said he would bring a message of U.S. support to both capitals.
Pope Francis urged the EU to come up with creative ways to stay together following Britain's vote, saying it's clear "something isn't working in this unwieldy union."
"The European Union must rediscover the strength at its roots, a creativity and a healthy disunity, of giving more independence and more freedom to the countries of the union," the pontiff told reporters as he flew home from Armenia.
The key, he said, is to rekindle the will to stay together with "creativity and new life."
Associated Press writers Bradley Klapper in Rome, Jill Lawless in London and Nicole Winfield aboard the papal plane contributed to this report.
[Editor notes: Eds: Corrects typo to health from heath. With AP Photos. AP Video.]