PARIS -- France's political mainstream, shut out of the presidency by an angry electorate, united Monday to urge voters to back centrist Emmanuel Macron in the presidential runoff and to reject Marine Le Pen's populist nationalism.

Politicians on the moderate left and right, including French President Francois Hollande and the losing Socialist and Republicans party candidates in Sunday's first-round vote, manoeuvred to block Le Pen's path to power in the May 7 runoff.

In a solemn address from the Elysee palace, Hollande said he will vote for Macron, his former economy minister, because the far-right Le Pen represents "both the danger of the isolation of France and of rupture with the European Union."

Hollande said the far-right would "deeply divide France" at a time when the terror threat requires "solidarity.

"Faced with such a risk, it is not possible to remain silent or to take refuge in indifference," he said.

European stock markets surged, and France's main index hit its highest level since early 2008, as investors gambled that the rise of populism around the world -- and its associated unpredictability in policymaking -- may have peaked.

Voters narrowed the French presidential field from 11 to two on Sunday. The contest is widely seen as a litmus test for the populist wave that last year prompted Britain to vote to leave the European Union and U.S. voters to elect Donald Trump president.

Only the defeated far-left candidate, Jean-Luc Melenchon, pointedly refused to back Macron.

Le Pen's far-right National Front party, meanwhile, is hoping to peel away voters historically opposed to a party long tainted by racism and anti-Semitism.

"The voters who voted for Mr. Melenchon are angry voters. They can be in agreement with us," National Front Vice-President Steeve Brios told The Associated Press, adding that those far-left voters sought choices "outside the system."

Choosing from inside the system is no longer an option. Voters rejected the two mainstream parties that have alternated power for decades in favour of Le Pen and the untested Macron , who has never held elected office and who founded his own political movement just last year.

Macron's optimistic vision of a tolerant France and a united Europe with open borders is a stark contrast with Le Pen's darker, inward-looking "French-first" platform that calls for closed borders, tougher security, less immigration and dropping the shared euro currency to return to the French franc.

Le Pen went on the offensive against Macron in her first public comments Monday.

"He is a hysterical, radical 'Europeanist.' He is for total open borders. He says there is no such thing as French culture. There is not one domain that he shows one ounce of patriotism," she said.

Le Pen's father, Jean-Marie, made it into a presidential runoff against Jacques Chirac in 2002 and was crushed. Many commentators expect the same fate for his daughter, but she has already drawn far more support than he ever did and she has transformed the party's once-pariah image.

National Front vice-president Louis Aliot insisted that Le Pen offers an alternative for anyone skeptical of the EU and France's role in it.

"I'm not convinced that the French are willing to sign a blank check to Mr. Macron," he said.

But Macron's party spokesman, Benjamin Griveaux, scoffed at the idea that Le Pen is a vector of change.

"She's been in the political system for 30 years. She inherited her father's party and we will undoubtedly have Le Pens running for the next 20 years, because after we had the father, we have the daughter and we will doubtless have the niece," he said, referring to Marion Marechal-Le Pen. "So she is in a truly bad position to be talking about the elites."

German Chancellor Angela Merkel wished Macron "all the best for the next two weeks."

Merkel's chief of staff, Peter Altmaier, tweeted that "the result for Emmanuel Macron shows: France AND Europe can win together! The centre is stronger than the populists think!"

Macron came in first in Sunday's vote, with just over 23 per cent while Le Pen had 21 per cent. Francois Fillon, the scandal-plagued conservative Republicans candidate, came in third with just shy of 20 per cent of the vote, just ahead of Melenchon. Socialist candidate Benoit Hamon, whose party holds a majority in the legislature, got just 6 per cent of the vote.

Turnout for Sunday's vote was 78 per cent, down slightly from 79 per cent in the first round of presidential voting in 2012.

Protesters burned cars, danced around bonfires and dodged riot police overnight at the Place de la Bastille and Republique in Paris. Twenty-nine people were detained at the Bastille, where protesters waved red flags and sang "No Marine and No Macron!" in anger at the results.

"We are in a phase of decomposition, demolition, deconstruction," former Socialist Prime Minister Manuel Valls said. "We didn't do the work -- intellectual, ideological and political -- on what the left is, and we paid the price."

Elaine Ganley reported from Henin-Beaumont. Lori Hinnant, Thomas Adamson, Angela Charlton et Philippe Sotto contributed from Paris.

A previous version of this story has been corrected to show that the Socialist candidate was Benoit Hamon, not Manuel Valls.