French president Francois Hollande has urged voters to choose centrist candidate Emmanuel Macron in the May 7 presidential run-off to keep out far-right leader Marine Le Pen.
Speaking from the Elysee palace, Mr Hollande said Ms Le Pen's platform of pulling out of the euro would devastate the country's economy and threaten French liberty.
He said the far-right would "deeply divide France" at a time when the terror threat requires solidarity and cohesion.
Mr Macron was Mr Hollande's top adviser on economic issues from 2012 to 2014, then economy minister in his Socialist government for two years.
In April 2016, he launched his own political movement, En Marche! (In Motion!) to prepare his presidential bid as an independent centrist candidate. He quit the government a few months later.
Mr Hollande's intervention came as France's defeated political mainstream united to urge voters to back Mr Macron.
Politicians on the moderate left and right, including the Socialist and Republicans party losers in Sunday's first-round vote, sought to block Ms Le Pen's path to power.
The mainstream parties were shut out of the presidency after the first round, which narrowed the presidential field from 11 to two.
This election is widely seen as a litmus test for the populist wave which last year prompted the UK to vote to leave the European Union and led to Donald Trump being elected US president.
The defeated far-left candidate, Jean-Luc Melenchon, pointedly refused to back Mr Macron, and Ms Le Pen's National Front is hoping to do the once-unthinkable and gain the support of voters historically opposed to a party long tainted by racism and anti-Semitism.
National Front vice president Steeve Brios said: "The voters who voted for Mr Melenchon are angry voters. They can be in agreement with us."
He said that these voters can express a choice "outside the system".
Choosing from inside the system is no longer an option, as voters rejected the two mainstream parties which have alternated power for decades in favour of Ms Le Pen and the untested Mr Macron, who has never held elected office and who founded his own political movement just last year.
Turnout was 78%, down slightly from 79% in the first round of presidential voting in 2012.
Socialist candidate Benoit Hamon, whose party holds a majority in the legislature, received just 6% of the vote. Socialist president Francois Hollande, the most unpopular in modern French record-keeping, did not seek re-election.
Former Socialist prime minister Manuel Valls said: "We are in a phase of decomposition, demolition, deconstruction.
"We didn't do the work - intellectual, ideological and political - on what the left is, and we paid the price."
Francois Fillon, the scandal-plagued conservative Republicans candidate, fared marginally better, coming in third with just shy of 20% of the vote.
Both centre-right and centre-left fell in behind Mr Macron, whose optimistic vision of a tolerant France and a united Europe with open borders is a stark contrast to Ms Le Pen's darker, inward-looking "French-first" platform, which calls for closed borders, tougher security, less immigration and dropping the euro to return to the French franc.
Ms Le Pen went on the offensive against Macron in her first public comments on Monday.
She said: "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."
Mr Macron's party spokesman, Benjamin Griveaux, said that Ms Le Pen is hardly a vector of change.
"She's been in the political system for 30 years," he said.
"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 and the people."
Mr Macron came in first in Sunday's vote, with just over 23%; Marine Le Pen had 21%; Mr Melenchon and Mr Fillon each had 19%. Mr Fillon, a former prime minister, bested the former Trotskyist Mr Melenchon by just 94,998 votes.
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 the associated potential unpredictability in policy-making - may have peaked.
German chancellor Angela Merkel also wished Mr Macron "all the best for the next two weeks".