French voters turned out in number on Sunday for the second round of a historic election that is expected to leave the far right as the biggest force in a deeply divided parliament.

At midday, according to interior ministry figures, some 26.63 percent of voters had turned out for the high-stakes poll -- the highest number at that time since 1981.

The mood in France is tense, with 30,000 police deployed to head off trouble and voters anxious about a potential electoral earthquake shifting the French political landscape.

In the village of Rosheim, outside the eastern city of Strasbourg, an "anguished" 72-year-old Antoine Schrameck said he feared France would see "a turning point in the history of the republic".

And in Tourcoing, near the northeast city of Lille, 66-year-old retiree Laurence Abbad said she feared violence after the results are announced. "There's so much tension, people are going mad," she said.

President Emmanuel Macron called the snap elections three years ahead of time after his forces were trounced in June's European parliament vote, a gamble which seems to have backfired.

Far right leader Marine Le Pen's National Rally (RN) came top in the June 30 first round, and is on course to repeat the feat in Sunday's run off races.

But she may not win the outright majority that would force Macron to appoint Le Pen's lieutenant, the RN party leader Jordan Bardella, 28, as prime minister just weeks before Paris hosts the Olympics.

A hung parliament with a large eurosceptic, anti-immigration contingent could weaken France's international standing and threaten Western unity in the face of Russia's invasion of Ukraine.

European Union officials, already learning to deal with far-right parties in power in Italy and the Netherlands, are watching France closely.

And in Rome, Pope Francis chose the day of the French vote to warn against "ideological temptations and populists", adding: "Democracy is not in good health in the world today."

With the country on tenterhooks, last week saw more than 200 tactical-voting pacts between centre and left wing candidates in seats to attempt to prevent the RN winning an absolute majority.

This has been hailed as a return of the anti-far right "Republican Front" first summoned when Le Pen's father Jean-Marie faced Jacques Chirac in the run-off of 2002 presidential elections.

Opinion polls now forecast that the RN will fall well short of the 289 seats needed for an outright majority in the 577-seat National Assembly, while still becoming the largest party.

Such an outcome could allow Macron to possibly build a broad coalition against the RN and keep Gabriel Attal as prime minister on a caretaker basis.

But it could also herald a long period of paralysed politics in France, as it prepares to host the Olympics from July 26.

"Today the danger is a majority dominated by the extreme right and that would be catastrophic," Attal said in a final pre-election interview with French television on Friday.

Many in France remain baffled over why Macron called an election that could end with the RN doubling its presence in parliament and his contingent of centrist MPs halving in number.

But the president, known for his theatrical gestures, appears intent on executing what he calls a "clarification" of French politics, which he hopes will eventually leave three clear camps of far right, centre and hard left.

The final opinion polls published by two organisations on Friday projected the RN would win between 170 to 210 seats, followed by the New Popular Front (NFP) broad left-wing coalition on 145 to 185 and Macron's centrists on 118 to 150.

While Macron's Ensemble alliance is forecast to come third, the more successful NFP is a fragile mix of several warring factions ranging from traditional Socialists to the hard-left France Unbowed (LFI) of firebrand Jean-Luc Melenchon.

"France is on the cusp of a seismic political shift," said analysts at the European Council on Foreign Relations (ECFR), warning that even if Macron controlled the government after the election he would face "legislative gridlocks".

This would weaken "France's voice on the European and international stage".

After voting began on Saturday in France's overseas territories, polls opened in mainland France at 0600 GMT and were due to close by 1800 GMT.

Projections -- which usually give a very close idea of the final outcome -- are published shortly afterwards, with the political leaders then reacting rapidly to any frenzy that grips the nation. (AFP)