At the Place d'Italie in the south of the capital Paris, gangs of youths hurled paving stones and bottles at riot police, who responded with tear gas.
A cameraman was hit in the chest as the youths hurled missiles at reporters covering the protests, until a group of plain clothes police officers intervened, making arrests. There were protests in other cities.
Police said 312 people were arrested nationwide, including 205 in Paris, while nine police officers were slightly injured in incidents on the sidelines of the marches.
Organisers said more than three million people took part in the demonstrations, while police set the figure at just over one million.
Thousands of officers were on standby across the country to prevent a repeat of the violence and vandalism that marred previous days of action against the government's First Employment Contract (CPE).
Incidents also broke out in Lille, in the north, where hundreds of youths hurled missiles at riot police, smashed shop windows and vandalised cars.
A female television reporter was assaulted while filming the violence, while a young man suffered burn injuries from the shell of a teargas grenade.
Riot police in northwestern Rennes fired teargas at demonstrators after they were targeted with bottles and stones, as youths set garbage cans on fire and smashed bus shelters and windows.
Firefighters said they had evacuated at least three people, including one hit with a rubber bullet.
And in the northern city of Caen, a photographer was injured in clashes between police and demonstrators who had set up a roadblock on the city's ring road.
’Economy threatened’
Meanwhile, the head of the MEDEF employers' organisation said the youth employment crisis, coming just months after rioting gripped poor suburbs across the country, is a threat to France’s economy.
"This is the second major crisis faced by our country in the last six months: the suburban riots in November and the crisis over the First Employment Contract (CPE) in March and April.
"We need to realise that this endangers the country's economy," MEDEF chief Laurence Parisot said in an interview on the LCI news channel.
Between one and three million people on Tuesday took part in a fifth day of nationwide strikes and protests against the controversial new employment law.
Mr Parisot, whose group gave only lukewarm support to Prime Minister Dominique de Villepin's job scheme, has warned repeatedly that the crisis is harming France's image abroad.
The CPE, which was conceived as a tool to tackle youth unemployment, which runs at 22 percent in France, is a contract for under 26-year-olds that can be terminated by the employer without explanation during a two-year trial period.
It has provoked a massive popular backlash, with opponents accusing Mr Villepin of trampling on hard-won labour rights and railroading the measure through parliament without due consultation with unions and employers.
The measure was signed into law over the weekend but immediately suspended by President Jacques Chirac while a new version is drafted in an attempt to end the crisis.