French voters have overwhelmingly rejected the European Union's proposed constitution in a key referendum.
The "No" camp on both the right and the left is jubilant
Almost 55% of people voted "No", with 45% in favour. Turnout was high, at about 70%.
The vote could deal a fatal blow to the EU constitution, which needs to be ratified by all 25 member states.
President Jacques Chirac accepted the voters' "sovereign decision", but said it created "a difficult context for the defence of our interests in Europe".
The French leader had campaigned hard for a "Yes" vote.
The BBC's Caroline Wyatt in Paris says the rejection by one of the EU's founding members is a political earthquake, sending reverberations right across Europe.
Winners and losers
European Commission President Jose Manuel Barroso described the French rejection as "a very serious problem," but insisted: "We cannot say that the treaty is dead."
But the BBC's world affairs analyst Paul Reynolds says there is no Plan B for the EU and no clear way forward, with moves towards further integration now in doubt.
In his televised address, Mr Chirac said voters had registered their frustrations.
He said he would take a decision on the future of his centre-right government "in the coming days", hinting at a possible reshuffle.
Late on Sunday jubilant "No" supporters gathered at the Place de la Bastille in Paris - where the French Revolution began - chanting "we won" and sounding horns.
Those who rejected the treaty came from across the political spectrum, including Communists, dissident socialists and right-wing parties.
One of the leading right-wing opponents of the treaty, Philippe de Villiers, said: "Europe has to be rebuilt. The constitution is no more."
He urged Mr Chirac either to stand down or dissolve parliament.
The leader of opposition Socialist party Francois Hollande, from the "Yes" campaign, voiced "regret" over the result, but blamed Prime Minister Jean-Pierre Raffarin.
"The rejection of this treaty is above all the rejection of the government," he said.
Foreign Minister Michel Barnier said the vote was a "real disappointment" - but urged other EU countries to go ahead with their own votes.
ALREADY RATIFIED TREATY
Member states can ratify the document through a referendum or by parliamentary vote. So far nine countries have formally endorsed it.
Eight national referendums are still to come, including one in the Netherlands on Wednesday, where the "No" side is leading the opinion polls.
Prime Minister Jan Peter Balkenende called on his fellow Dutch to approve the treaty despite France's rejection.
"There is all the more reason to say 'Yes' so that some progress can be recorded," he said.
In the UK, Foreign Secretary Jack Straw said the result raised "profound questions" about the direction of Europe.
The constitution was finalised last year after long negotiations among EU governments.
Its aim is to streamline EU institutions following the admission of 10 new members last year.