Marine Le Pen's far-right National Front have stunned France's political elite by taking first place in European Parliament elections, with President Francois Hollande's Socialists beaten into a poor third, provisional results showed.
It was the first time the anti-immigrant, anti-EU party had won a nationwide election in its four-decade history.
If the results are confirmed, it could secure as many as 25 seats in the new European Parliament, more than eight times the three it won in 2009.
"The people have spoken loud and clear," a triumphant Le Pen told cheering supporters at party headquarters in a northwestern suburb of Paris.
National Front leader Marine Le Pen called for the dissolution of the French National Assembly after exit polls showed her anti-immigrant, Eurosceptic party winning European Parliament elections in France.
"What else can the president do after such a rejection?" Ms Le Pen said.
"It is unacceptable that the assembly should be so unrepresentative of the French people."
French voters have dealt a severe blow to Francois Hollande's Socialist government in local elections.
The anti-immigration far right, which claims that France's large Muslim population is "Islamicising" the nation, made solid advances, fulfilling National Front promises to begin building a grass-roots base.
Socialist leaders conceded defeat in the final round of the voting seen as a referendum on unpopular President Hollande, who was expected to reshuffle the cabinet in an effort to give his government a boost.
Polls suggest the French far-right National Front party of Marine Le Pen has failed to win biggest prizes - the city of Perpignan.
Exit polls suggest the French far right have failed to win one of their biggest prizes of the night: Perpignan.
Exit polls suggest candidates backed by France's far-right Front Nationale party are on track to claim victories in the southern towns of Beziers and Frejus in the local elections.
The FN already made a breakthrough in last week's first round of voting by winning power in the northern town of Henin-Beaumont.
France's far-right Front Nationale party appears to have claimed a second town in the second round of local elections in which they are expected to make significant gains.
With polls in some areas already closed, ITV News Europe Editor James Mates has tweeted:
Front Nationale candidate looks to have won the southern town of Beziers, the 2nd to fall to the far right in these elections #france
The anticipated success in Beziers contrasted with a defeat elsewhere for the party of Marine Le Pen, which took an outright victory in the northern town of Henin-Beaumont in the first round.
Looking as if the Front Nationale have been soundly beaten in Avignon by the socialists. More results expected in 20 mins or so. #france
Despite that success, the governing Socialists - the party of President Francois Hollande - are braced for big losses while the centre-right UMP is expected to do well.
France's anti-immigrant National Front (NF) party has scored gains in first-round town hall elections and taken control of a former Socialist bastion as voters turned against President Francois Hollande and his left-wing allies.
The elections in thousands of constituencies across France were the first nationwide voter test for Hollande. A second round of voting is due next Sunday.
FN leader Marine Le Pen, who took over from her father Jean-Marie Le Pen in 2011, said advances made in the first round already marked a major breakthrough.
While only fielding candidates in 596 out of some 36,000 municipalities across France, BVA estimated the FN had scored seven percent of the vote.
FN's candidate in the northern town of Henin-Beaumont, a former coalmining centre that had long been held by Socialists, was declared winner with an outright majority.
France's National Front party is threatening legal action after one of Madonna's videos appeared to show its leader and a swastika.Read the full story ›
Marine Le Pen, who was knocked out in the first round of the country's presidential elections, casts her vote in Henin Beaumont.