Centrist Emmanuel Macron on Sunday became France's youngest President with a stunning win over far-right rival Marine Le Pen, a verdict loudly cheered by the European Union.

Wild celebrations broke out in many parts of Paris and across France as Macron, 39, wrapped up an overwhelming 66.06 per cent of all votes in a closely and bitterly fought battle. This amounted to 20.7 million votes.

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A civil servant who became a millionaire investment banker and then a government minister, Macron founded the 'En Marche!' movement and went on to become the President despite not having a constituency of his own and without the backing of any major party.

But Le Pen, 48, took about 10.6 million votes in the Presidential election's crucial runoff or 33.94 per cent of the votes. This was almost double the tally her more rabid father Jean-Marie won in 2002, the last time a far-right candidate made it to the runoff. 

She told her National Front supporters that the new divide in France was between "globalists and patriots".

As Le Pen called up Macron to congratulate him, the new President vowed to fight "the forces of division that undermine France", media reports said.

In a speech to jubilant supporters, Macron said: "Tonight you won, France won. Everyone told us it was impossible. But they don't know France." 

He tweeted earlier: "As of this evening and for the coming five years, I am going to serve with humility, with dedication, with determination, in your name."

Macron acknowledged the anger of many voters and pledged to protect the weakest members of the society.

"I will work to renew the links between Europe and its citizens. My duty is to alleviate fears and rekindle optimism," he added. "I will fight with all my strength against the divisions that separate us." 

European Union leaders heaved a sigh of relief over the defeat of Le Pen, who had threatened a "Brexit" if she came to power.

European Commission chief Jean-Claude Juncker said: "Happy that the French chose a European future."

German Chancellor Angela Markel tweeted her congratulations: "Macron's win was a victory for a strong united Europe."

US President Donald Trump, who had earlier praised Le Pen, congratulated Macron for the "big win" and said he looked forward to working with him.

Indian Prime Minister Narendra Modi also hailed Macron's victory.

Russian President Vladimir Putin, who had openly backed Le Pen, welcomed Macron's victory. He urged Macron to bridge "deep rifts" and work together in the face of growing threats of terrorism and violent extremism.

Macron supporters admitted that he would face huge challenges, with a third of the electorate choosing Le Pen or abstaining or casting a blank ballot, the French and European media said.

It was also clear that the tradition Left had voted for Macron — not because they supported him but because they feared chaos if Le Pen were to capture the presidency.

With nearly 47 million registered voters, the turnout, excluding French overseas territories, was registered at 75.34 percent in the second round, lower compared to 78.69 per cent in the April 23 round and 80.35 per cent in the 2012 runoff.

Macron's first significant political challenge would be June's legislative elections.

Macron has just weeks to go before taking the reins of government and engineering a parliamentary majority in the National Assembly.

He is expected to meet stiff resistance from the National Front, led by defeated second-round presidential candidate Le Pen, and the Republicans party.