Whoever wins the poll - whether it's the independent centrist Emmanuel Macron or the far-right's Marine Le Pen - the next French president will inherit a country bitterly divided.
On May 5, dozens of students protested outside of 10 high schools in Paris, holding signs that read, "Neither Le Pen nor Macron, neither the fatherland nor the boss", a reference to the nationalist stances of Le Pen and the pro-business Macron. "His big victory confirms that a very large majority of our fellow citizens wanted to gather around the values of the Republic and mark their attachment to the European Union, " Hollande said in a statement.
A recent voter survey found that seven in 10 voters are unhappy with the choice between Macron and Le Pen.
Some 47 million people are eligible to vote.
Voting was not due to end until 8 p.m., but Belgian media published what they said were surveys taken on Sunday by four unnamed pollsters among people who had voted or meant to vote. An information blackout entered into force shortly after.
The unusually tense and unpredictable French presidential campaign ended with a hacking attack and document leak targeting Macron on Friday night.
France has started voting in the final act in one of the most tumultuous presidential election campaigns in the country's history.
As First Lady, she will assume a powerful role that Mr Macron's aides compare to that of Michelle Obama during her time in the White House.
"The world is watching", said 32-year-old marketing worker Marie Piot as she voted in a working-class part of northwest Paris.
But after polls were wrong about Brexit and the USA election, investors are wary.
For many years Marine Le Pen has been outspoken on Islamic extremism and has promised the French that she will bring it "to its knees".
Florence, 49, accountant, in Britain for 22 years: "I voted Macron in the first round and will do the same today".
Tense France votes in presidential election, decides Europe's fate
France picks its new president Sunday.
Global financial markets and France's neighbours are watching carefully.
The two candidates got through the first round two weeks ago, and now go head-to-head for the French presidency.
Outgoing French president Francois Hollande and prime minister Bernard Cazenveuve both voted around 10:25 (0825GMT).
He left the Socialist government in August and formed En Marche!
The ex-investment banker's programme pledges to cut state spending, ease labour laws, boost education in deprived areas and extend new protections to the self-employed.
Macron said Le Pen stirred up the hatred and the anger of voters the way her father did, calling her "the high priestess of fear". His big idea is that, by showing that France is capable of serious internal reforms, the country will be able to persuade Germany to shift the European Union toward a less austere economic policy, one more favorable to growth.
At rallies or in interviews, Le Pen evokes only victory, saying, "When I am president".
Le Pen, whose National Front party received a multimillion-dollar loan from a Russian bank in 2014, has defended Russia's annexation of Ukraine's Crimean Peninsula and has met several times with Russian President Vladimir Putin - most recently in Moscow in late March. She advocated the withdrawal of France from the common European currency.
Le Pen has also vowed to reduce net immigration to 10,000 people a year, crack down on outsourcing by multinationals, lower the retirement age and introduce hardline measures to tackle Islamist extremists.
Far-right candidate Marine Le Pen faces centrist Emmanuel Macron after voters bucked France's traditional two-party system in April's first round of the election. The move was criticized on social media, with some users blasting the promotion as a sign of "no more democracy".
Le Pen's camp heavily criticized Macron for his celebrations after the first round of voting, labeling him as arrogant.
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