President Chirac appealed for calm today after mass protests against youth job reforms erupted in violence across France, and police prepared for even bigger demonstrations at the weekend.

M Chirac called for tomorrow's trade union and student marches - which are expected to draw up to a million people - to take place in a climate of "calm and respect for all" and for talks to begin as soon as possible.

At an awards ceremony at the Elysee Palace, M Chirac appealed to all to exercise responsibility to avoid a repeat of the violence that erupted at the end of yesterday's student demonstrations.

Between 250,000 and half a million people marched in 80 towns and cities across France in a show of force over a new youth job contract, denounced as a licence to hire and fire at will by students, unions and the left-wing opposition.

Police made 300 arrests overnight, two-thirds of them in central Paris, where gangs of youths - described as outside troublemakers - fought running battles with police, vandalising cafes and burning bookshops.

Several hundred people overturned cars and hurled firebombs at riot police on the capital’s Place de la Sorbonne, as police repelled them with tear gas and water cannons, until calm returned early on Friday.

Ninety-two police officers and 18 demonstrators were injured in the clashes, according to police, and several officers were also hurt in street violence in the northern city of Rennes, where police made 19 arrests.

Nicolas Sarkozy, the Interior Minister, blamed yesterday’s violence on "thugs" and "hooligans" from the far-left and far-right, as well as young delinquents from the Paris suburbs, who he said had infiltrated the march in the capital.

"There were a few hundred delinquents out looking for a fight," M Sarkozy told reporters after meeting police and firefighters in Paris overnight. Seventy-seven people remained in custody as Pascal Clement, the Justice Minister, told prosecutors to be "firm".

Dominique de Villepin, the Prime Minister who championed the First Employment Contract (CPE), faces the most serious test of his short premiership over the wave of protests. He has repeatedly said that he is open to talks but insists the measure - adopted by parliament last week as part of a package of laws drawn up after last year’s suburban riots - will be implemented.

Opponents of the CPE, a first job contract for under-26-year-olds which can be broken off without explanation in the first two years, say it will entrench job insecurity.

Unions organising tomorrow's protests are riding a tide of public support, with 68 percent of respondents in a new poll saying they want the government to withdraw the measure.

M Chirac repeated his defence of the scheme, saying it was an "important element in fighting unemployment", which affects one in four French youths, and tops 50 percent in some of the high-immigration suburbs hit by riots last year.

He argued that the contract would create new jobs for young people who are largely left by the wayside in the jobs market.

"It brings opportunities and new guarantees for young people in difficulty," M Chirac said. "The government, as you know, is ready for dialogue, and for my part I wish it to begin as soon as possible".

Strikes and sit-ins have spread to two-thirds of France’s 84 universities as well as dozens of high schools - drawing a growing rumble of protest from students angry at the disruption to their studies.

Around 100 students from universities across Paris demonstrated in the Latin Quarter today to demand an end to the student blockades of their faculties, while anti-strike student groups announced plans for a counter-demonstration on the Place de la Sorbonne next week.

The wave of protests has revived memories of the May 1968 student uprising, and have been seen as the sign of a deeper malaise among young French people worried about their future.