More than 1,000 police raided housing projects outside Paris in an early morning sweep Monday, detaining 33 people in a bid to find rioters who led an outburst of violence here last year, a prosecutor said.

Police were mobilized for raids in Villiers-le-Bel and in the neighboring towns of Sarcelles, Gonesse and Arnouville as part of the investigation into the November riots, according to police.

Marie-Therese de Givry, prosecutor of Pontoise, said a total of 33 people were arrested, raising the number from some 20 cited earlier by police.

Most of those detained, aged 19 to 31, had been known to police, mainly for previous violence, according to police.

In one raid, about 100 police officers surrounded one building in Villiers-le-Bel across from a library and pre-school that were burned down by rioters. It was not immediately clear whether there were arrests in that building.

Violence erupted Nov. 25-26 in Villiers-le-Bel, populated largely by families of immigrant backgrounds, after two teenage boys were killed in a motorbike crash with a police car. Police and local officials said it was an accident, but many residents were unconvinced.

The flare-up in Villiers-le-Bel stoked fears of broader unrest like the three weeks of riots across the nation's neglected suburbs two years earlier, in November 2005. Many of those rioters were Arab or black, French-born children or grandchildren of immigrants from France's former colonies who were frustrated by entrenched discrimination and isolation.

In Monday's raid, police were targeting about 40 people suspected of attacking officers during last year's violence, a police official said on condition of anonymity. The official was not authorized to speak to the media. Two gang leaders in particular were targeted, according to police, who said they organized the unrest, telling others where to stand or how to attract police into poorly lit areas.

During the unrest, 130 officers were injured, including at least 10 when rioters fired shotguns at them.

Prosecutors in the nearby city of Pontoise opened a judicial inquiry into attempted homicide against the assailants. Investigators issued an appeal for witnesses and promised monetary rewards for information leading to the shooters.

Monday's raids comes after President Nicolas Sarkozy announced a sweeping plan earlier this month to better integrate poor suburban youth and tackle the racism they often face in the job market — and to better police their neighborhoods.