Over 50,000 police backed by elite units of the French security services patrolled the streets less than three days after a suspected Islamist gunman shot dead a policeman and wounded two others on the central Champs Elysees avenue.
Voters will decide whether to back a pro-EU centrist newcomer, a scandal-ridden veteran conservative who wants to slash public spending, a far-left eurosceptic admirer of Fidel Castro or to appoint France's first woman president who would shut borders and ditch the euro.
The outcome will show whether the populist tide that saw Britain vote to leave the EU and Donald Trump's election in the United States is still rising, or starting to ebb. A high level of indecision adds to nervousness.
Hanan Fanidi, a 33-year-old financial project manager, was still unsure as she arrived at a polling station in Paris' 18th arrondissement.
"I don't believe in anyone, actually. I haven't arrived at a candidate in particular who could advance things. I'm very, very pessimistic," she said.
Emmanuel Macron, 39, a centrist ex-banker who set up his party just a year ago, is the opinion polls' favourite to win the first round and beat far-right National Front chief Marine Le Pen in the two-person run-off on May 7.
For them to win the top two qualifying positions on Sunday would represent a huge change in the political landscape. The second round would then feature neither of the mainstream parties that have governed France for decades.
But conservative Francois Fillon is making a comeback after being plagued for months by a fake jobs scandal, and leftist Jean-Luc Melenchon's ratings have surged in recent weeks. Any two of the four has a chance to qualify for the run-off.
"It wouldn't be the classic left versus right divide but two views of the world clashing," said Ifop pollsters' Jerome Fourquet. "Macron bills himself as the progressive versus conservatives, Le Pen as the patriot versus the globalists."
The seven other candidates, including the ruling Socialist party's Benoit Hamon lag behind in opinion polls.
By noon (1000 GMT), turnout amid sunshine and clear skies across much of France was 28.54 percent, according to official figures - around the same as in the 2012 first round, in which almost 80 percent eventually took part.
Some polls had been predicting a much lower turnout, closer to the 70 percent that took the then National Front leader Jean-Marie Le Pen into the second round in 2002. Pollsters are unclear about what a low or high turnout could mean in 2017.
President Francois Hollande and his predecessor Nicolas Sarkozy have failed through both of the past two five-year presidencies to tackle the high unemployment and sluggish growth.
That issue, and the general trustworthiness of politicians, stands out, polls say, even though security has re-entered the debate since Thursday's killing of a policeman.
Some argue the incident increases Le Pen's chances; but previous militant attacks, such as the November 2015 killing of 130 people in Paris ahead of regional polls, have not appeared to have any impact on votes.
Earlier on Sunday a polling station in Besancon, eastern France was evacuated after a stolen vehicle was abandoned with the engine running while voting took place.
The possibility of a Le Pen-Melenchon run-off is not the most likely scenario but is one which alarms bankers and investors.
While Macron wants to further beef up the euro zone, Le Pen has told supporters "the EU will die". She wants to return to the Franc, re-denominate the country's debt stock, tax imports and reject international treaties.
Melenchon also wants to radically overhaul the European Union and hold a referendum on whether to leave the bloc.
Le Pen or Melenchon would struggle, in parliamentary elections in June, to win a majority to carry out such radical moves, but their growing popularity also worries France's EU partners.
"It is no secret that we will not be cheering madly should Sunday's result produce a second round between Le Pen and Melenchon," German Finance Minister Wolfgang Schaeuble said.
If either Macron or Fillon were victorious, each would face challenges.
For Macron, a big question would be whether he could win a majority in parliament in June. Fillon, though likely to struggle less to get a majority, would likely be dogged by an embezzlement scandal, in which he denies wrongdoing.