Other parties, from communists to President Nicolas Sarkozy's conservatives, rallied behind Daniel Duquenne whom voters designated the new mayor of Henin-Beaumont, a former mining town in northern France, in a runoff race.

The victor was sprayed with tear gas minutes after the results were announced, a police officer said by telephone, confirming reports on France-Info radio and the French TV station iTele. Duquenne was not injured and the aggressor or aggressors fled, said the officer who was not authorized to discuss the situation and demanded anonymity.

Police patrolled the streets of Henin-Beaumont, and planned to maintain their presence throughout the night, the officer said.

Far-right National Front candidate Steeve Briois had won last week's first-round vote with a 20-point margin, but parties fearing a return of the far right banded together to block him in the final round of the bi-election.

Duquenne's victory was slim but comfortable, with 52.4 percent of the vote, compared to Briois' 47.6 percent.

The barrage of support for Duquenne recalled the successful bid to block National Front leader Jean-Marie Le Pen in the 2002 presidential race against President Jacques Chirac.

However, the National Front was not about to go down easily. Briois announced plans to ask the administrative court to cancel the election results, saying Duquenne won "dishonestly." Briois claimed that his opponent told various media outlets that the city of 26,000 would lose state subsidies if the National Front won.

The Henin-Beaumont election was organized after Socialist Mayor Gerard Dalongeville was jailed in early April on preliminary charges of alleged extortion and favoritism. He was excluded from the Socialist Party.

"I'm happy with what my political family did for this election," a top aide to Sarkozy, Henri Guaino, said on French television, referring to the Union for a Popular Movement, or UMP, to band with Socialists and other rivals against the National Front. "This is the triumph of democracy."

The anti-immigration party, once a feared political force in France, never fully recovered from Le Pen's crushing defeat in the runoff of the 2002 presidential race as parties on the left and right joined to defeat him.

Losses in the legislative elections and the 2007 presidential vote sent the anti-immigration party spiraling into debt, and it was forced to sell its headquarters last year.

The far-right won four southern towns in 1995, but later lost them. It looked to Henin-Beaumont, just south of Lille, as a new chance to put its mark on France's political map.

Marine Le Pen, daughter of the National Front leader and his likely successor, saw victory in the defeat with the relatively close results. "We're not losers in this vote," she said on French television. "The results are a starting point for the National Front of tomorrow."

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Cecile Brisson in Paris contributed to this report.