French voters are choosing their next president in an election that could determine if the European Union will survive or not.
The second round contest pits centrist Emmanuel Macron, a 39-year-old former investment banker, against the far-right nationalist Marine Le Pen, 48.
Citizens in some overseas territories and many French expats abroad have begun voting.
The polls opened in metropolitan France at 08:00 local time (06:00 GMT) on Sunday and close at 19:00 (17:00 GMT).
Polling stations will remain open in some big cities until 20:00 local time (18:00 GMT), with early estimates of the result due to be reported immediately after they close.
The two candidates, who topped a field of 11 presidential hopefuls in the first round election on 23 April, have offered voters starkly different visions of France.
Mr Macron, a liberal centrist, is pro-business and a strong supporter of the European Union (EU), while Ms Le Pen campaigned on a France-first, anti-immigration programme.
She wants France to abandon the euro in the domestic economy, and hold a referendum on France’s EU membership.
Mr Macron is widely expected to win the vote, but analysts have said high abstention rates could damage his chances.
The run-off will be keenly watched across Europe, ahead of elections in Germany and the UK and as Britain negotiates its exit from the EU.