By Michel Rose and Leigh Thomas
PARIS (Reuters) - France's mainstream political parties were scrambling for a way to stop the rise of the far-right National Front (FN) on Monday after its historic first-round lead in regional elections.
Boosted by fears over the Islamic State militant attacks that killed 130 people in Paris on Nov. 13, Marine Le Pen's party secured 27.7 percent of the vote nationally.
The National Front came first in six of 13 regions in Sunday's vote, its best showing ever. The anti-Europe, anti-immigration party has been gaining ground for years among voters frustrated by the government's failure to tackle unemployment, fearful of immigration and disillusioned by mainstream politics.
Riding a wave of euroscepticism and anti-immigrant feeling which has brought far-right parties to prominence across Europe, the breakthrough bolsters Le Pen's position as a serious contender for the 2017 presidential election.
Some 16 percent of those who voted for the FN said they had changed their voting intentions after the Nov. 13 attacks, an exit poll published on Monday said.
Sixty-eight percent said their aim had been to punish the unpopular Socialist government of President Francois Hollande, the poll by Ifop and Fiducial for iTELE, Paris Match and Sud Radio said.
The outcome also exposed fault lines within both the country's main traditional political groupings over the right tactics to confront the National Front in the decisive second round of the regional elections next Sunday.
To try to make sure Le Pen does not win in the final round, France's ruling Socialist Party decided to pull its candidates out of three regions where it came third, telling supporters there to back Nicolas Sarkozy's conservative Republicans.
But many in President Francois Hollande's party are unhappy about giving up all regional representation across swathes of the country, and on Monday, Jean-Pierre Masseret, who leads the Socialists in the eastern region, resisted calls from his party chief to pull out.
"We are standing fast. We think the best way to oppose the National Front is by taking our seats in the regional assembly. That is where we can best push back the National Front, by being the opposition," he said on BFMTV.
French Prime Minister Manuel Valls, who had not spoken publicly since the results announcements, said on Monday evening Masseret should withdraw his list.
"There is no point in holding to it," he said in an interview with private national French television channel TF1.
"We must live up to what is at stake and that is to strengthen the Republic," he added.
Sarkozy has ruled out a similar tactic by his own party, but some of his allies believe he should copy the Socialists' strategy.
"When you are third, you pull out. You create a front against the destructive force because now is the time to rebuild," said former prime minister Jean-Pierre Raffarin, who is from the centrist rank of the party.
Long the pariah of French politics, the FN has won greater respectability since Marine took over from her father Jean-Marie as head of the party in 2011.
While it won 11.42 percent of the vote in the first round of the last regional elections in 2010, it got 25.25 percent in the first round of elections to the smaller departments in March.
Le Pen senior was thwarted in his bid for the presidency in 2002 when the Socialist Party used a similar tactic to the one they deployed on Monday. Its own candidate, Lionel Jospin, was knocked out in round one, so it urged supporters to prevent a Le Pen win by voting for Conservative Jacques Chirac, who won with more than 80 percent of the vote.
On Monday, Marine Le Pen was careful not to claim victory, and also denounced the Socialists' tactics as anti-democratic.
"We're not home and dry yet, especially since the election is being run in an unfair way," she told French radio RTL.
"To pull out a candidate on the second round is rather unfair. Today, they're twisting the arm of the French, of the left. Are they going to accept being insulted, despised? We'll see," she said.
The National Front won 27.73 percent of the first-round vote nationwide, according to final official figures. The Republicans and its allies won 26.65 percent and the Socialists 23.12.
Le Pen won 40.64 percent of the vote in the Nord-Pas-de Calais region she contested and her niece Marion Marechal-Le Pen scored 40.55 percent in Provence-Alpes-Cote D'Azur in the south. The abstention rate was 50.9 percent nationally.
(Additional reporting by Matthias Blamont; Writing by Andrew Callus; Editing by Janet Lawrence and Toby Chopra)