Britain's vote to become the first country to leave the European Union, as projected by national media, is a shattering blow that threatens the survival of the post-war European project, officials and analysts said.
The loss of one of its biggest members will at the very least force major changes on an embattled bloc already struggling to deal with growing populism, a migration crisis and economic woes.
In the long-run, "Brexit" may lead to other countries holding referendums, a far looser union, and possibly even the disintegration of a grouping set up 60 years ago to bring security and prosperity after World War II.
EU President Donald Tusk warned in the run-up to the vote that Brexit could lead to the "destruction of not only the EU but also of Western political civilisation."
With Europe facing a resurgent Russia and the threat of terrorism, Mr. Tusk said "our enemies... will open a bottle of champagne if the result of referendum is negative for us."
In a less doom-laden assessment, European Commission Chief Jean-Claude Juncker said last week that the EU was not "in danger of death" from a Brexit but that it would have to learn lessons.
'Very serious blow'
Chris Bickerton, a lecturer at Britain's Cambridge University and author of "The European Union: A Citizen's Guide", said it was a "very serious blow" but not terminal, given the "core role" of the EU in much of European political life.
But he added that it would probably drift towards a "looser, ad hoc" union.
"I don't think it would suddenly disappear but over the longer term, we might see it slowly decline and become something different," he told AFP.
The next steps for the EU would be difficult, he added.
"We are very much in uncharted territory," he said. "I don't think anyone really thought Brexit was really likely, certainly not when they were negotiating with Cameron, otherwise they would have done a very different deal."
In the immediate aftermath of the British vote, seven years of potentially bitter divorce negotiations between Brussels and London loom.
The remaining EU countries will likely be keen to move ahead. France and Germany, the main EU heavyweights, have already been working on a joint plan for the future.
But with Berlin and Paris at loggerheads over future integration of the eurozone, any plan is likely to be a modest affair that deals only with issues such as security and defence.
Even without Britain in the club, the drift away from "ever closer union" and federalism is likely to increase, with growing talk of a "two-speed Europe" that allows states opt-outs from key rules.
One major step could be making membership of the euro non-compulsory, which would help Poland, which appears to have no intention of joining the single currency but is officially meant to.
Domino effect?
The main fear in many European capitals is that either way, the result could trigger a domino effect of referendums in other countries.
French far-right leader Marine Le Pen on Tuesday urged all EU states to follow Britain's example, and eurosceptics in the Netherlands, Denmark and Sweden have made similar calls for referendums.
Vivien Pertusot, Brussels-based analyst with the French Institute of International Relations (IFRI), said the EU was likely to survive but be weakened.
"Institutions rarely die," he told AFP. "Maybe there will not be disintegration, but a loss of relevance. The EU will lose, bit by bit, its centrality for all the most political projects."
The danger for the EU is that even after if makes changes following the British referendum, it will still not be able to quell the forces of history tearing it apart.
"The EU is in a negative spiral," Janis Emmanouilidis, director of studies at Brussels-based think tank European Policy Centre, told AFP.
The question of what could replace the EU if it does collapse is even more vexed.
"It might sound as if yes, this story has ended, a new one has began, but that's not not easy. Especially after the experience of failure," said Emmanouilidis.
Here are the next steps after "Brexit":
SAVE THE MARKETS
With the stability of the global economy at risk, early Friday the European Central Bank is widely expected to make a statement to reassure markets.
TOP EU OFFICIALS REACT
The EU's top officials meet in Brussels at 0830 GMT. EU President Donald Tusk, European Commission chief Jean-Claude Juncker, European Parliament head Martin Schulz and Netherlands Prime Minister Mark Rutte, whose country holds the six-month EU presidency, are expected to make a statement and address the press at around 1000 GMT.
MINISTERS TACKLE BREXIT
The EU's 28 European Affairs Ministers meet in Luxembourg to lay the groundwork for Brexit talks at the EU summit on June 28-29. Talks start at 1230 GMT.
GERMAN SOLUTIONS
Foreign ministers from the EU's six founding countries -- France, Germany, Italy, Belgium, Netherlands, Luxembourg -- are expected to meet on Saturday in Berlin, according to European sources.
French President Francois Hollande will meet with German Chancellor Angela Merkel next week to discuss "European initiatives", expected on Monday.
FRANCO-GERMAN INITIATIVE
The Merkel Hollande meet-up could be the occasion to announce plans for a long-rumoured Franco-German initiative on a better integrated defense and security strategy for Europe. The leaders intend to use the plan to shore-up doubts on the EU project unleashed by the British vote campaign.
BRUSSELS ACTS
On Monday the European Commission's top officials, who are nominated by the EU's 28 member states, begin mapping out the long road to an official Brexit at an extraoridiary meeting in Brussels.
-- MEPs have called for an extraordinary session of European Parliament to be held on Monday also in Brussels in the case of a Brexit vote.
THE 'BREXIT' SUMMIT
The 28 EU leaders -- still including British Prime Minister David Cameron -- meet on June 28 and 29 in Brussels to digest and debate the results of the Thursday Leave vote. It was originally due to be held on June 23 but was postponed after the British referendum date was announced.
ENTER SLOVAKIA
On July 1, the Netherlands hands over the EU's six-month rotating presidency to the relatively inexperienced Slovakia, which now must lead the negotiations towards Brexit. Britain had been due to take the helm at the end of 2017 but will now give that up.
CRUEL SUMMER
EU civil servants delay summer holidays to begin the painstaking legal work to bring about Brexit. If confirmed, the process to break the UK away from Europe will take at least two years.
