LONDON — First the party, then the hangover.
As the U.K.'s unprecedented decision to leave the European Union reverberated worldwide, officials from the bloc's six original founding members called for exit negotiations to begin immediately.
France's Foreign Minister Jean-Marc Ayrault said it would not be acceptable for Britain to play a "cat and mouse" game.
"Of course a new (British) prime minister must be appointed, that will probably require a few days but this is quite urgent," he said on Saturday after an emergency meeting attended by Germany, France, Italy, the Netherlands, Belgium and Luxembourg.
"It's in Britain's interest and in the interest of Europeans not to have a period of uncertainty that would have financial consequences, and that could have economic and political consequences," Ayrault added.
The vote to leave the EU forced British Prime Minister David Cameron to announce he would step down by the fall.
The country now "must prepare for a negotiation" with the European Union, one that "will require strong, determined and committed leadership," he told reporters outside his Downing Street office Friday. "I do not think it would be right for me to be the captain that steers our country to its next destination."
Talks should start "as soon as possible," said Germany's Foreign Minister Frank-Walter Steinmeier, also in Berlin.
European Central Bank Governing Council member Francois Villeroy de Galhau warned London's financial center would lose its prized "EU passport" if Britain fails to negotiate continued access to the bloc's single market.
Banks based in London rely on the so-called EU passport to work across the Europe's markets unhindered while basing much of their staff and operations outside the euro zone. Britain has maintained a separate currency, the pound, while most of Europe adopted the euro.
"If tomorrow Britain is not part of the single market, the City cannot keep this European passport, and clearing houses cannot be located in London either," Villeroy, who is also governor of the French central bank, told France Inter radio.
The French capital has been a rival to the City, as London's financial center is known.
Meanwhile, Britons were taking in that the fact that their anti-establishment vote had pushed their economy into uncertain territory, sparking a crisis for a bloc founded to unify post-war Europe.
Many were clearly unhappy with the decision, with a petition calling for a second referendum garnering more than two million signatures.
The U.K.'s surprise move triggered huge selloffs in stock markets worldwide with the Dow Jones Industrial Average dropping more than 600 points on Friday.
"Leave" campaigners have hailed the result as a victory for British democracy against the bureaucratic behemoth of the EU.
Conservative former London Mayor Boris Johnson, who has made no secret of his ambition to take over at Number 10 Downing Street, said "the British people have spoken up for democracy in Britain and across Europe."
Nigel Farage, leader of the hard-right U.K. Independence Party, said "the dawn is breaking on an independent United Kingdom."