Far right voters may decide who becomes France's next president after anti-immigration crusader Marine Le Pen's record first-round election score.
Her performance has complicated the race between Socialist frontrunner Francois Hollande and incumbent Nicolas Sarkozy.
The centre-left Mr Hollande narrowly beat the conservative Mr Sarkozy in yesterday's 10-candidate first round by 28.6% to 27.1%, the Interior Ministry said, but Ms Le Pen stole the show by surging to 18%, the biggest ever result for a far-right candidate.
Her breakthrough mirrored advances by anti-establishment Eurosceptical populists from Amsterdam and Vienna to Helsinki and Athens as anger over austerity, unemployment and bailout fatigue deepen due to the eurozone's grinding debt crisis.
"The battle of France has only just begun," Mr Le Pen, 43, daughter of former paratrooper and National Front founder Jean-Marie Le Pen, told cheering supporters last night.
Declaring that her wave of support was "shaking the system" of mainstream consensus politics, she said: "Nothing will be the same again."
Ms Le Pen, who wants France to abandon the euro, said she would give her view on the runoff at a May Day rally in Paris next week.
But she saved most venom for Mr Sarkozy, aiming to pick up the pieces in any recomposition of the right and hoping the Front can enter parliament in June.
National Front Vice-President Louis Alliot suggested this morning that Ms Le Pen would not formally endorse either candidate "as things stand".
"Based on the ideas in our programme, neither one defends or develops them, so it seems unlikely," he said.
Amid strong turnout, more than one third of voters cast ballots for protest candidates outside the political mainstream.
Mr Sarkozy, the first sitting president to be forced into second place in the first round of a re-election bid, will have to attempt a difficult balancing act to attract both the far-right and centrist voters he needs to win the 6 May runoff.
After five years of leading the world's fifth economy, he could go the way of 10 other eurozone leaders swept from office since the start of the crisis in late 2009.
Mr Hollande, 57, who opinion polls taken yesterday showed winning the decider with between 53 and 56% of the vote, vowed to change the direction of Europe if elected and lead an economic revival with greater social justice.
"My final duty, and I know I'm being watched from beyond our borders, is to put Europe back on the path of growth and employment," he told supporters in his constituency of Tulle in southwestern France.
Financial market analysts say whoever wins in two weeks' time will have to impose tougher austerity measures than either candidate has admitted during the campaign, cutting public spending as well as raising taxes to cut the budget deficit.
A parliamentary election to be held in June will further determine the complexion of the next French government.
