The French Interior Ministry said the No camp had 57.26%, compared to 42.74 for the Yes camp.
French Foreign Minister Michel Barnier said the country's rejection of the EU constitution in a national referendum was "a real disappointment".
"This is an ordeal, a real disappoint-ment," Barnier said on TF1 national television shortly after the release of exit polls showing a massive defeat of the landmark EU charter at the ballot box.
Defence Minister Michele Alliot-Marie called the result on France 2 television a "defeat for France, a defeat for Europe".
The vote was also a political slap in the face for President Jacques Chirac who had staked his prestige on the campaign for the treaty.
There was much at stake for the 72-year-old president, for whom the rejection of the treaty is a big setback two years before presidential and parliamentary elections.
The constitution sets rules for the EU that are intended to make decision-making easier after the bloc's enlargement from 15 to 25 member states in May 2004.
But many French voters regarded the referendum as a chance to punish Chirac and his conservative government for unemployment at a five-year high of 10.2% and other economic problems.
Some people voted 'no' because
of the high unemployment level
"I voted 'no' in all conscience, having read the text, due to the lack of will to solve Europe's number one problem today, which is unemployment," said Armel Bompart, 52, a civil servant in Strasbourg, home to the European parliament.
Others such as Burgundy restaurateur Jose Robalo said he changed his mind at the last minute to back the constitution despite deep misgivings about European integration.
"I have disagreed with Europe from the start but we are stuck with it and I am thinking of my children - perhaps it will be better for them," Robalo said behind the bar of his restaurant in the central city of Dijon.
France has some 42 million registered voters, with around one in five undecided when campaigning ended on Friday.
Voters in overseas territories cast their ballots on Saturday.
Chirac had said he would not quit but could react to a "no" vote by dismissing unpopular Prime Minister Jean-Pierre Raffarin.
Interior Minister Dominique de Villepin, a trusted ally, leads the race to replace him.
Earlier, Aljazeera's correspondent in Paris, Ayash Darraji, said many French people felt that the European constitution focused too heavily on economic issues at the expense of social ones.
He said the consequences of a "no" vote would be serious, and would undermine France's position in Europe and Chirac's position in his own country.
Pointing out that France was the tenth country to vote on the EU constitution and the second to run a referendum on the issue, Darraji said France's attitude could influence the other EU members.
Other EU members had voted on the issue through their national parliaments, he added.
Meanwhile, in the Netherlands
, where voters are widely expected to reject the constitution in a referendum on Wednesday, leaders appealed to the Dutch not to heed the result in France
"It's up to the French what they decide but we Dutch must make up our own minds, just as we've always done and will continue to do," Foreign Minister Bernard Bot told Dutch television before the French results were out, saying the treaty was good for Netherlands
. Tough negotiations
The European constitution was signed by EU leaders last October in Rome
after long and tough negotiations and requires the approval of all member states to go into force.
So far, nine countries have approved it and none have rejected it.
"If we want to be strong enough to face up to the United States, we have to vote for Europe"
If it were thrown out, the EU would continue to operate under current rules widely seen as unworkable for a union intent on enlarging further, and voting could soon become paralysed.
Supporters said the treaty would have helped make Europe and France stronger in the face of economic threats from other countries such as China. They said a French "no" would kill the treaty.
"Europe is the future. If we want to be strong enough to face up to the United States, we have to vote for Europe. We've got the euro, now we need to coordinate the rest," treaty supporter Rachel Allouche said outside a Paris polling station.
Opponents said the treaty enshrined economic policies that had failed to stop the loss of jobs to low-wage economies, including countries outside the EU.
"Even if it is a bit irresponsible to vote 'no', we have to show we are against the government's current policies. We want a society that puts people at the centre. This vote is the common people speaking," countered "no" voter Francois Lequeux.
They say a "no" vote would force the EU to redraft the treaty and improve it.
Source: Aljazeera + Agencies