Update 1.35pm: Emmanuel Macron's campaign press office said the courtyard outside the Louvre museum in Paris where the Presidential candidate has planned to celebrate election night has been evacuated because of a security alert.
Campaign spokeswoman Pauline Calmes said the Esplanade du Louvre was evacuated as a precaution.
She did not specify the nature of the threat but says police ordered the evacuation.
Voters across France are casting ballots in a presidential election run-off that could decide Europe's future, choosing between independent Emmanuel Macron and far-right populist Marine Le Pen.
With Mr Macron the pollsters' favourite, voting stations opened across mainland France at 8am under the watch of 50,000 security forces guarding against extremist attacks.
Polling agency projections and initial official results will be available when the final stations close at 8pm.
The unusually tense and unpredictable campaign ended with a hacking attack and document leak targeting Mr Macron on Friday night.
France's government cybersecurity agency is investigating the hack.
Either candidate would lead France into uncharted territory since neither comes from the mainstream parties that dominate parliament and have run the country for decades.
The fate of the European Union (EU) may hang in the balance as France's 47 million voters decide whether to risk handing the presidency to Ms Le Pen, who dreams of quitting the bloc and its common currency, or to play it safer with Mr Macron, an unabashed pro-European who wants to strengthen the EU.
A so-called Frexit would be far more devastating than Britain's departure since France is the second-biggest economy to use the euro.
The country also is a central pillar of the EU and its mission of keeping post-war peace via trade and open borders.
The vote will help gauge the strength of global populism after the victories last year of a referendum to take Britain out of the EU and Donald Trump's US presidential campaign.
In France, it is a test of whether voters are ready to overlook the racist and anti-Semitic past of Ms Le Pen's National Front party.
Ms Le Pen has broadened the party's appeal by tapping into - and fuelling - anger at globalisation and fears associated with immigration and Islamic extremism.
Mr Macron has argued that France must rethink its employment laws to better compete globally and appealed for unity and tolerance.
The winner will have to try to build a parliamentary majority in elections next month to make major changes.
Voting began yesterday in overseas territories, from Saint-Pierre-et-Miquelon, an archipelago near Newfoundland, to French Guiana and the French West Indies and beyond.
French citizens also turned out in droves to vote in the Canadian province of Quebec.
Fears of outside meddling hung over the race after France's election campaign commission said yesterday that "a significant amount of data" - and some fake information - was leaked on social networks following the hacking attack on Mr Macron.
The leaked documents appeared largely mundane and the perpetrators remain unknown.
The commission urged French media and citizens not to relay the leaked documents.
French electoral laws impose a weekend news blackout on any campaigning and media coverage seen as swaying the election. Ms Le Pen's campaign could not formally respond due to the blackout.