Front National Front party candidate, Laurent Lopez (L), waves to supporters as he arrives with French National Front party deputy Marion Marechal-Le Pen (R) and his wife Caroline (C) after winning the second round of the local by-election in Brignoles October 13, 2013. REUTERS/Jean-Paul Pelissier

By Sybille de La Hamaide

PARIS (Reuters) - France's National Front (FN) won by a wide margin in a local by-election in southeast France on Sunday, the latest sign the anti-immigrant party is benefiting from discontent with ruling Socialists and mainstream opposition conservatives.

Even though the National Front already has an established presence in the region, the victory in the canton of Brignoles near the port town of Toulon suggests it is well placed to make gains in 2014 municipal and European Parliament elections.

FN candidate Laurent Lopez won 53.9 percent of votes against 46.1 percent for center-right UMP candidate Catherine Delzers, according to final results.

President Francois Hollande's Socialists did not field their own candidate in the first round, instead backing a Communist ally who failed to make it into Sunday's run-off. They subsequently called on left-wing voters to back the UMP but even that was not enough to avert an FN victory.

"This victory with an unambiguous score and a record turnout demonstrated that the National Front is a unifying party," Lopez said after the results were announced.

The FN has long attracted protest votes, but leader Marine Le Pen is increasingly expanding her appeal to disgruntled Socialist and UMP voters with tough talk on crime and illegal Roma immigrants. She has also capitalized on the government's unpopularity and UMP's deep divisions.

"This should serve as a lesson to the left as a whole," Socialist Party spokesman Eduardo Rihan-Cypel told LCI Television.

A poll earlier this week of voting intentions for next May's European Parliament elections put the anti-EU party ahead of France's two big mainstream parties for the first time in any nationwide vote.

Hollande's popularity has fallen nearly continuously since he was elected last year, largely on his failure to stem a rise in unemployment and a leadership style often judged as hesitant.

Only 26 percent were satisfied with his performance in a monthly OpinionWay poll for Clai-Metro-LCI released on Sunday, down 3 points on September and just off a low of 23 percent in July.

Sunday's election was the third attempt in three years to elect a district representative for Brignoles, which has a high population of North African-origin families in its center.

The FN had won the ballot in 2011 but France's highest administrative court invalidated the results seen as too tight. The following year, the Communist candidate had the upper hand but results were again annulled for the same reasons.

(Additional reporting by Marine Pennetier; editing by Mark John)