France's beaten mainstream parties have lined up behind Emmanuel Macron to try to stave off a victory for the far-right's Marine Le Pen in the final round of the presidential election.
The Republicans' François Fillon and Socialist Benoît Hamon urged supporters to vote for Mr Macron on 7 May.
EU heavyweights have also backed Mr Macron, the favourite in opinion polls.
Ms Le Pen quickly turned her fire on her rival, calling him a "weakling" for his anti-terrorism policies.
The victory of the two candidates in Sunday's first round meant that, for the first time in six decades, neither of France's main left-wing or right-wing parties had a candidate remaining in the election.
Conceding defeat, Mr Fillon, third on Sunday with 19.9%, said there was "no other choice than voting against the far-right".
Mr Hamon, who represented the Socialists of unpopular President François Hollande and who was trounced into fifth with just 6.4%, said voters had to keep Ms Le Pen - "an enemy of the republic" - from power.
However, the position of the fourth-placed candidate, hard-left Jean-Luc Mélenchon, who took a creditable 19.6%, was still unclear.
He pointedly refused to back Mr Macron, whose pro-EU, pro-business and pro-globalisation stance is diametrically opposed to his anti-austerity, eurosceptic approach.
Steeve Briois, vice-president of Ms Le Pen's National Front, said it was hoping to win over Mr Mélenchon's supporters, who he called "outside the system".
"The voters who voted for Mr Mélenchon are angry voters. They can be in agreement with us," Mr Briois told the Associated Press.
However, protesters who burned cars at the Place de la Bastille and Place de la Republique in Paris overnight were chanting "No Marine and No Macron!"
The latest opinion poll, by Opinionway, suggested a second-round victory for Mr Macron by 61% to 39%.
But there were warnings from Mr Macron's own party following a glitzy victory celebration at a Paris bistro that the job was not yet done.