Incumbent Nicolas Sarkozy arrived at a Paris polling station on Sunday to cast his vote in the country's presidential run-off.
He was accompanied by his wife Carla Bruni-Sarkozy.
Socialist candidate Francois Hollande is expected to defeat Sarkozy by capitalising on public anger over the government's austerity policies.
Under Sarkozy, France pledged to rein in its spending while the rest of 17 countries that use the euro embark on a strict period of belt-tightening.
Hollande has promised more government spending and higher taxes - including a 75-percent income tax on the rich - and wants to re-negotiate a European treaty on trimming budgets to avoid more debt crises of the kind facing Greece.
A recent poll by the BVA agency showed 52.5 percent support for Hollande and 47.5 percent for Sarkozy.Polling stations in France will opened from 8 a.m. (0600 GMT) on Sunday to 6 p.m. (1600 GMT), and two hours later in big cities.
Reliable projections of the result based on a partial vote count will be published as soon as the last polling stations close. Media that publish exit polls or partial results before that risking fines and legal action.
The election comes at a crucial time for the convalescent euro zone, as France, Europe's No. 2 economy, is a vital partner for Berlin in safeguarding the single currency bloc's future. Nicolas Sarkozy was headed for an election defeat that could make him the 11th European leader to be swept from office by the economic crisis and crown Francois Hollande as France's first Socialist president in 17 years.
Buoyed by a tide of anger over Sarkozy's inability to rein in rampant unemployment during his five-year term, Hollande was between four and eight points ahead in final opinion polls for a vote that could mean a shift in direction for Europe.
Despite shaving a couple of points off Hollande's lead in the last days of a frenetic campaign, the conservative's own aides privately admitted it would require a miracle for him to turn the odds in his favor and clinch a second term.
Reuters