Polls have opened in the French presidential run-off election between Emmanuel Macron of En March and far-right leader Marine Le Pen after a tumultuous campaign filled with scandal and surprises.

Key points: Opinion polls indicate French voters will pick Emmanuel Macron

Opinion polls indicate French voters will pick Emmanuel Macron 50,000 security forces will be guarding against extremist attacks

50,000 security forces will be guarding against extremist attacks Polling agency projections, initial official results expected at 8:00pm (local time)

Some 67,000 polling stations across mainland France will be under the watch of 50,000 security forces guarding against extremist attacks.

Security is a prime concern in the wake of a series of militant attacks in Paris, Nice and elsewhere in the past few years.

Polling agency projections and initial official results are expected as soon as the final stations close at 8:00pm local time (4:00am AEST).

Opinion polls indicate French voters will pick Mr Macron, a 39-year-old ex-economy minister who wants to bridge the left-right divide, resisting an anti-establishment tide that has seen Britons vote to leave the EU and Americans choose Donald Trump as US president.

But should an upset occur and National Front candidate Marine Le Pen win, the very future of the EU could be on the line.

Mr Macron, who wants to deregulate the economy and deepen EU integration, has a 23 to 26 percentage point lead over Ms Le Pen in the opinion polls.

Forecasts proved to be accurate for the presidential election's first round last month, and markets have risen in response to Mr Macron's widening lead over his rival after a bitter debate on Wednesday.

The campaign ended on Friday with a hacking attack and document leak targeting Mr Macron.

France's government cybersecurity agency, ANSSI, is investigating the hack, which Mr Macron's team says was aimed at destabilising the vote.

En Marche said the documents released online only showed the normal functioning of a presidential campaign, but authentic documents had been mixed on social media with "false documents" to sow "doubt and misinformation".

Huge majority not backing Macron 'wholeheartedly'

The first round of voting saw Mr Macron edge Ms Le Pen by 2 percentage points. ( Reuters: Eric Gaillard )

Ms Le Pen, who wants to close borders, ditch the euro currency and clamp down on migration, is nevertheless closer to elected power than the far right has ever been in Western Europe since World War II.

Even if opinion polls prove accurate and France elects its youngest president rather than its first female leader, Mr Macron has admitted he expects no honeymoon period.

Abstention could be high, and close to 60 per cent of those who plan to vote for Mr Macron say they will do so to stop Ms Le Pen from being elected rather than because they fully agree with the former banker-turned-politician.

"The expected victory … wouldn't be a blank cheque for Emmanuel Macron," Odoxa pollsters said in a note.

"A huge majority will not be backing him wholeheartedly."

The election will not end the battle between mainstream and more radical policies in France, with parliamentary elections next month equally crucial.

Reuters/AP