All voting stations will close by 8 p.m. (2 p.m. ET) Sunday, and polling companies release usually reliable projections of the final result almost immediately afterward.
France is suffering from high unemployment, a stagnant economy and security worries. The government has struggled to cope with immigration and integration.
By 5 p.m. Paris time (11 a.m. ET), 65.3% of registered voters had cast their ballots. That turnout is down from the last election day in 2012, when almost 72% of registered voters had cast ballots by 5 p.m..
A high abstention rate is likely to hit Macron harder than Le Pen, analysts have said.
In the election's first round two weeks ago, voters rejected representatives of all the traditional mainstream political parties
in France, with Macron and Le Pen topping a field of 11 candidates by taking 24% and 21% of the vote, respectively.
The two-round election, which has played out like something of a soap opera
, was hit with another scandal at the eleventh hour, when Macron's campaign announced it had been the target of a "massive and coordinated" hacking operation
Around 14.5 gigabytes of emails, personal and business documents
were posted to the text-sharing site Pastebin just hours before the campaign period came to a close Friday night.
Macron's party said the hackers had mixed fake documents with authentic ones "to create confusion and misinformation." It is not clear who was behind the attack.
Le Pen has spent the past few weeks battling to extend her appeal beyond her traditional base of supporters, while Macron has been attempting to convince voters that he is not part of the political elite they rejected in the first round.
Macron, 39, has campaigned on a pro-Europe, pro-integration platform. Le Pen, 48, has suggested she would aim to take France out of the European Union, withdraw it from NATO and forge closer ties with Russia.
Security on voters' minds
Voters in the capital city of Paris braved heavy rains to get to polling stations. At a town hall in the city's 18th district, a group of nuns from the Benedictine Sisters of the Sacred Heart in Montmartre was among morning voters.
Pascal Bardin, 52, described the election as crucial, saying the future of Europe rested on the vote.
"Depending on how it goes, this vote could threaten global security, national stability and our values," Bardin told CNN.
Macron voted in the northern city of Le Touquet, where he and his wife, Brigitte Trogneux, greeted supporters with handshakes and kisses. Le Pen cast her vote in the heartland of Henin-Beaumont, also in the country's north, with her partner, Louis Aliot.
French President Francois Hollande cast his ballot in the southwestern city of Tulle. He made the unusual decision not to run for a second term, as his approval ratings have sunk in recent years following a spate of deadly terrorist attacks.
He spoke to journalists outside the polling station, saying that France had overcome many challenges and would continue to do so under a new president.
"We must always have a road ahead. It is this road that makes us France -- we will never go backwards, we will always move forwards, looking for the right road for progress."
Hollande presided over the country during the 2015 Paris attacks
, the deadliest terror attack on French soil in its modern history, in which 130 people were killed.
The country is still under a state of emergency following those attacks and several others. Some 12,000 extra police and soldiers are on duty in the capital for election day to secure polling stations and the candidates' headquarters, Paris police said.
There appeared to be a security alert at the Louvre in Paris in the early afternoon, as police cleared journalists from an area outside the art museum's famous glass pyramid.
Macron's camp has booked the courtyard there to hold a rally after the polls close, and hundreds of journalists are accredited to cover the event.
Paris police tried to play down the sweep, saying they were scouring the scene to check there was "nothing dangerous," in what they said was a "precaution."
Campaign gets dirty
The campaign period ahead of the final round has had its dirty moments.
Both candidates traded insults in a bad-tempered, head-to-head debate
on French television on Wednesday. Macron called Le Pen a liar who sowed division and hatred, while she accused him of being soft on terrorism and said he would preside over a nation enfeebled by its powerful neighbor, Germany.
Less than 24 hours after the debate, the Paris prosecutor opened a preliminary investigation
after Macron filed a complaint against Le Pen following her claim during the debate that he may have an offshore account in the Bahamas.
Macron, a former investment banker, who also served as economy minister under Hollande, has struggled to connect with voters in the rural and de-industrialized
areas of the country.
He was upstaged in his own hometown of Amiens, when Le Pen made a surprise visit to a Whirlpool factory
at threat of closure to rally support while Macron met with union representatives in the same city.
Le Pen's camp heavily criticized Macron for his celebrations after the first round of voting, labeling him as arrogant.
Will voters abstain?
In the final polls published before campaigning ended on Friday, Macron appeared to have retained a healthy lead. But the unknown quantity is turnout: A campaign launched last week urged voters to stay at home, leave their ballot envelope empty or submit a blank piece of paper instead of a ballot slip. It is unclear if the steady voter turnout in the morning will continue through the day.
While the first-round turnout was relatively healthy, official government figures show more people abstained in the April 23 vote than the number who voted for any single candidate -- including Macron and National Front's Le Pen.
The big challenge for Le Pen has been to broaden her appeal. At the end of last month, she announced that she had temporarily stepped down from her position as leader of the National Front. Some saw that as an attempt to distance herself from the party, regarded as toxic by many in France.
But her position in the polls has barely moved since the first round. If Le Pen is elected, it would be one of the biggest shocks in postwar French political history.