A divided France voted Sunday in a referendum on the European Union's first-ever constitution, with opinion polls pointing towards a "no" that would send a shockwave across the continent.

Voters appeared to be turning out in high numbers, with midday participation at 25 percent, four percentage points higher than at the same hour in 1992 when France went to the polls on the Maastricht treaty to create the euro.

After a hard-fought campaign that polarized the country, the last opinion polls released Friday indicated that opponents of the landmark EU charter would prevail, with between 51 and 56 percent of the vote.

But with about one in five French voters expected to make their decision at the last minute, "everything is possible", as pro-treaty Socialist partyleader Francois Hollande said after casting his ballot in southwest France.

Rejection of the constitution, which aims to simplify decision-making in the expanded 25-member bloc, would spark a period of deep political uncertainty both in the EU and in France, one of the Union's six founding members.

Other EU countries fear that a French "non" -- the first in the bloc -- would unleash a domino effect across the continent, influencing voters in the Netherlands, Poland, Denmark and Britain to say "no" in future referendums.

"We, the European people, will decide on our future. It's a democratic choice. That's good news in and of itself," Prime Minister Jean-Pierre Raffarin said after voting in his home region of Poitou, central France.

President Jacques Chirac -- whose political legacy is riding on Sunday's vote -- made no statements to the press after he voted in the central-southwest village of Sarran, but stopped to sign autographs and pose for photographs.

Polling stations were to remain open to the country's 42 million eligible voters until 8:00 pm (1800 GMT), with residents of Paris and Lyon given a further two hours to cast their ballots.

Exit polls were not to be released before 10:00 pm (2000 GMT), so as not to prejudice last-minute voters. Due to the time difference, the 1.4 million voters in France's overseas territories went to the polls on Saturday.

"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," said treaty supporter Rachel Allouche outside a Paris polling station.

"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.

Chirac's ruling Union for a Popular Movement (UMP) party campaigned for the "yes" alongside its junior partner in government, the Union for French Democracy (UDF), as well as the opposition PS and the Greens -- a coalition of France's mainstream political elite.

They faced a disparate "no" camp made up of the far-right National Front of Jean-Marie Le Pen, Communist and Trotskyist parties, nationalist Euroskeptics and former Socialist prime minister Laurent Fabius.

A come-from-behind victory for the "yes" camp would bolster Chirac, allow EU officials in Brussels to breathe a sign of relief and keep the EU treaty on track ahead of a consultative referendum in the Netherlands on Wednesday.

The French president, who has staked his prestige on approval of the treaty, warned voters that a French "no" would diminish the country's influence in Europe, making it the "black sheep" of the continent.

Rejection of the treaty would have a profound political impact in France, tarnishing Chirac's record in the history books, prompting a government reshuffle and deepening the rift in the Socialist party.

No matter what the outcome on Sunday, Chirac is expected to dismiss his unpopular prime minister, with Interior Minister Dominique de Villepin or Defense Minister Michele Alliot-Marie tipped to replace him.

Should France reject the treaty, Chirac -- who celebrated his 10th year as president earlier this month -- would be unlikely to seek a third term in office in 2007, opening the door to his arch-rival, UMP leader Nicolas Sarkozy.

Chirac has however ruled out his resignation should the "no" camp prevail.

The constitution must be ratified by all 25 member states. So far, nine EU countries have approved the treaty -- Austria, Germany, Greece, Hungary, Italy, Lithuania, Slovakia, Slovenia and Spain.