The French far-right National Front (FN) party has made strong gains in local elections.
The anti-immigration party won an outright majority in one northern town that had long been a stronghold of the Socialist Party President Francois Hollande.
The elections in thousands of constituencies across France were the first nationwide voter test for President Hollande, who came to power in May 2012.
He has seen his popularity slump to record lows for failing to rein in unemployment.
A second round of voting is due next Sunday, but FN leader Marine Le Pen said advances made in the first round already marked a major breakthrough.
"The National Front has arrived as a major independent force a political force both at the national and local level," said Ms Le Pen.
An exit poll put Mr Hollande's Socialists and their left-wing allies at 43% of the vote, trailing opposition conservatives whose 48% put them on track to reverse Socialist gains made in the 2008 municipal elections.
The FN scored 7% of the vote, a high national tally, given that it only fielded candidates in 596 out of the 36,000 municipalities across France.
Exit polls put the National Front ahead in the eastern town of Forbach in France's former industrial heartland.
In the south, the anti-EU party was in the lead in Avignon, Perpignan, Beziers and Frejus, and vying for second place in Marseille, behind the conservative incumbent.
Pollsters have identified half a dozen towns that could see FN rule after next Sunday's run-offs, giving it the chance to show it can be trusted with power after attempts to run four towns in the late 1990s revealed its lack of competence.
With official figures expected to show turnout at record lows of around 65% of voters, French Prime Minister Jean-Marc Ayrault made a television appeal for "all democratic forces" to close ranks against FN candidates next week.
"Wherever the FN is in a position to win the second round, all who support democracy and the Republic have a duty to prevent them," Mr Ayrault said.
Heavy losses for President Hollande's party could trigger a reshuffle of his cabinet and encourage backbench attacks on a raft of new pro-business policies, on which President Hollande has called a vote of confidence in coming weeks.
However, the final outcome will depend in some cases on highly unpredictable three-way races between the Socialists, the UMP and the National Front.