French Prime Minister Dominique de Villepin summoned eight ministers to his offices on Saturday to plan a response to the nightly clashes which, now in their second week, have increased in severity and spread to other French cities.

Nearly 900 vehicles were torched and over 250 people arrested on Saturday as French authorities feared those behind the country's worst rioting for decades were becoming organized.

Deprived suburbs with large immigrant populations on the fringes of Paris were again the scene of the worst of the rampages, which largely took the form of hit-and-run arson attacks.

Violence also flared in other cities around the country -- Lille and Rouen in the north, Rennes in the west, and Toulouse, Pau and Marseille in the south.

Interior Minister Nicolas Sarkozy and other security officials said they believed the gangs of hooded youths responsible were showing signs of organization, and were urging copycat acts via Internet blogs.

There were concerns over the fact the unrest was concentrated in neighborhoods with Muslim immigrants from France's former Arab and African colonial territories, a small proportion of whom have turned to radical Islam in the past few years.

A total of 253 people were detained for questioning on Saturday, some of them minors caught with fire-bombs, police said.

A government challenge

The violence began on October 27, sparked by the electrocution of two youths of African and Arab origins in the suburb of Clichy-sous-Bois after they hid from police in an electrical relay station.

But since then it has become a challenge to the authority of the government and a protest against the dismal economic prospects, rampant discrimination and heavy-handed policing that the youth in the suburbs suffer.

Local residents tired of the nightly chaos are also beginning to organize. In Aulnay-sous-Bois, a suburb of 80,000 people northeast of Paris, several thousand residents marched past the shells of burnt out vehicles with a banner reading "No to Violence, Yes to Dialogue."

"It's a sign that the laws of the republic apply to everyone and that we will not give in to violence," said mayor Gerard Gaudron, a member of the governing UMP party.

Calls for Sarkozy to step down

Sarkozy and his hardline rhetoric has become a focus for much of the anger, with the rioters vowing to keep up their actions until he is forced to resign.

The interior minister has been criticised for labelling delinquent youths "rabble" and declaring he would clean widespread crime out of the suburbs with "a power-hose."

His tough law-and-order policies over the past three years earned him the support of voters, who made him France's most popular politician -- a big boost for his ambition of running for president in 2007 elections.

But the rioting has badly tarnished his reputation, and opposition communist and Greens politicians have called for him to step down.

Sarkozy has rejected that path. Early Saturday he made a surprise visit to a police command centre west of Paris, and told officers: "Arrests -- that's the key."

He urged them to get more information on those causing the trouble "so we can better understand how they're organised, because they are organized."

Villepin is seeking to calm tensions, meeting with suburban youths and holding regular meetings with ministers and mayors to find ways to deescalate the situation, so far without success.