Francois Hollande (left) is mounting a strong challenge against President Sarkozy
Polls have opened in France's presidential election, taking place amid widespread concern over the eurozone crisis and high unemployment.
Centre-right incumbent Nicolas Sarkozy is seeking re-election, saying only he can preserve a "strong France".
But he is facing a tough challenge from Socialist Francois Hollande, who has said it is "the left's turn to govern".
There are 10 candidates in all, and if none wins more than 50% of the votes there will be a run-off round on 6 May.
Polls in mainland France and Corsica are open until 18:00 (16:00 GMT), with voting stations in big cities remaining open for a further two hours.
The first official results will be released after the last stations close at 20:00 (18:00 GMT).
President Sarkozy, who has been in office since 2007, has promised to reduce France's large budget deficit and to tax people who leave the country for tax reasons.
He has also called for a "Buy European Act" for public contracts, and threatened to pull out of the Schengen passport-free zone unless other members do more to curb immigration from non-European countries.
Mr Hollande, for his part, has promised to raise taxes on big corporations and people earning more than 1m euros a year.
He wants to raise the minimum wage, hire 60,000 more teachers and lower the retirement age from 62 to 60 for some workers.
If elected, Mr Hollande would be France's first left-wing president since Francois Mitterrand, who completed two seven-year terms between 1981 and 1995.
If Mr Sarkozy loses he will become the first president not to win a second term since Valery Giscard d'Estaing in 1981.
French presidents are now elected for five years.
Wages, pensions, taxation, and unemployment have been topping the list of voters' concerns.
But the candidates have been accused of failing to address the country's problems during a lacklustre campaign.
Frustration with Mr Sarkozy's flashy style and with Mr Hollande's bland image has also allowed radical candidates to flourish.
Marine Le Pen, a media savvy far-right leader, has invigorated her anti-immigration National Front.
Meanwhile Jean-Luc Melenchon, who is supported by the Communist Party, has galvanised far-left voters.
Centrist leader Francois Bayrou is standing as a presidential candidate for the third time. In 2007, he came third, with nearly 19% of the vote.
Voting was held on Saturday in France's overseas territories - including Martinique and Guadeloupe in the Caribbean, Reunion Island in the Indian Ocean and French Polynesia.
Those territories vote early because results will be known on Sunday evening in mainland France - when it is still mid-afternoon in Caribbean islands and other overseas territories.
The presidential vote will be followed by a parliamentary election in June.