Within minutes of the announcement of the result, gangs of young people apparently supporting the defeated Socialist candidate, Jean-Jacques Anglade, broke shop windows outside the town hall and clashed with riot police who dispersed them with tear gas. Mr Anglade appealed for calm.
According to the final results released by election officials, the National Front candidate for mayor, Catherine Megret, defeated Mr Anglade, by 52.48 per cent to 47.52 per cent.
In a statement, the extreme-right party declared: "The future belongs to us!" The FN leader, Jean-Marie Le Pen, hailed the victory as the "first swallow of a French spring". It would, he said, provide the breakthrough it needed for the Front to win seats in next year's French parliamentary elections.
The mainstream parties will write off the FN victory as a special case: a cry of protest from a depressed town with high racial tensions, disillusioned by a Socialist mayor facing charges of embezzlement. The problem is that many of these same issues - high unemployment, immigration and corruption in the political establishment - will provide fertile ground for the FN nationwide next year.
The psychological importance of Vitrolles was, in any case, admitted by the major parties, when they banded together to try to defeat the Front after Mrs Megret won the first round eight days ago. The centre-right withdrew its candidate, hoping to give a clear run to the widely loathed incumbent Socialist, Jean-Jacques Anglade. This strategy failed. It also handed an important propaganda victory to the FN.
The far right, reviled by all the other parties and almost the entire media as the enemy of Republican and democratic values, had defeated the combined ranks of the French establishment for the first time. The FN already controls three town halls in the south, but they all were won on the first-past-the-post system with less than 50 per cent of the electorate
The victory of Mrs Megret is also a triumph for her husband, Bruno Megret, the deputy leader of the Front National and possible successor to Mr Le Pen. Mr Megret, 47, could not run himself because he was banned after exceeding the spending limit in the last mayoral poll in Vitrolles in 1985, an election declared void because of widespread irregularities.
Last night's victory confirms Mr Megret as the second most powerful figure in the FN, a prospect which fills mainstream politicians and many in the Front with dismay. Although a highly intelligent man from a technocratic background, dissidents in the FN have accused Mr Megret of supporting more frankly racist and neo-Nazi positions than Mr Le Pen.
An atmosphere of violence hung over Vitrolles throughout the second round campaign. The tensions were inflamed before last night's clashes outside the town hall by the apparently deliberate running over of two town officials campaigning for the Socialist mayor. The officials were rammed by a car yesterday as they sat on a motorcycle just before polls closed. Three people were later arrested. According to one account, an axe, a spade and baseball bat were found in the boot of their vehicle.
All the leaders of the French left descended on Vitrolles in the last week to boost the mayor's credibility. They hoped the burst of national and international publicity surrounding the poll would persuade many anti-Front voters, who had not turned out last week, to support Mr Anglade. In the event, the centre-right vote split slightly in Mr Anglade's favour, but not enough to stop a FN victory.