PARIS — The upcoming parliamentary elections are the next test for France’s conservative, pro-American president-elect, Nicolas Sarkozy.

Under the constitution of the Fifth Republic, tailored back in 1958 for General Charles de Gaulle, the French president can wield considerable power if he has the backing of the National Assembly. If he does not, he must pick the leader of the Assembly’s largest party as prime minister. Overnight, he turns into a lame duck.

In 1986 and 1993, this happened to President Mitterrand, a socialist, who somehow managed to keep his prime ministers, Jacques Chirac and Edouard Balladur, at bay and survive. And between 1997 and 2002, the conservative President Chirac was forced to transfer effective power to a socialist prime minister, Lionel Jospin.

Mr. Sarkozy’s supporters are convinced that the June 10 and 17 parliamentary elections will turn in an “all blue” — staunchly conservative — National Assembly. Indeed, parliamentary ballots held on the heels of pro-president referendums or presidential elections in 1958, 1962, 1981, and 2002 turned in large pro-president majorities at the Assembly.

Things were less conclusive in 1988, however. Mitterrand had just been re-elected as president, but in the ensuing parliamentary ballot, his own party did not win an absolute majority. Socialist cabinets thus had to rely on communist or centrist support until 1993.

Urban unrest may become another major challenge for Mr. Sarkozy. The night after his Sunday victory was marred by large-scale violence reminiscent of the nationwide ethnic riots that rocked France in the fall of 2005 and again, more briefly, in the fall of 2006.

According to the French national police, 270 people were arrested, which suggests that thousands of rioters were involved. Three hundred sixty-seven cars were torched and about 30 police personnel were wounded.

Violence flared first in France’s second largest city, Lyons, when about 500 socialist sympathizers attacked a pro-Sarkozy victory party at 9 p.m. The fashionable Place Bellecour was vandalized and 200 cars burned.

At about the same time, similar incidents took place in Lille, Marseille, Toulouse (where left-wing hooligans desecrated the French flag), Bordeaux, Rennes, Caen, Metz, and Nancy. An hour later, violence erupted in Paris as 5,000 demonstrators gathered at Place de la Bastille, one of the city’s left-wing landmarks. Commando-style groups set cars aflame and engaged in street battles with the police. Most Paris suburbs also saw fires and clashes.

Moreover, there is a strong suspicion that the police are downplaying the violence and that many incidents are going unreported. Stories about the urban guerrilla operations that were carried out the night before the election have not appeared in the press, apparently at the behest of law enforcement officers. On Rue Lhomond in Paris’s Fifth Arrondissment, just a block from the picturesque Rue Mouffetard, several cars were torched at 5 a.m., producing flames so intense that the police considered evacuating the neighborhood. The state-run press agency, Agence France-Presse, failed to mention the incident on Election Day.

The rioters and urban guerrilla fighters are apparently linked to far left organizations or to the ethnic underworld, and they enjoy a measure of support in the Socialist Party and other left-wing parties. The defeated socialist presidential candidate, S้gol่ne Royal, insisted throughout the campaign that she stands for “bringing together” the various components of contemporary French society, and she portrayed Mr. Sarkozy as “divisive.” The current outburst of violence seems to bear out her warnings.

Mr. Sarkozy will succeed Mr. Chirac as president on May 10 and set up a temporary Cabinet that will be in charge of the parliamentary elections. If the urban violence continues, he will get the blame. If he crushes the rioters, he will be described as an “authoritarian” president and get the blame again. In both cases, quite a challenge.

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