More than one million people are estimated to have protested across France on Tuesday against the government's youth employment laws.
Fighting broke out as protesters gathered in Paris, and missiles were hurled at police as they moved into the crowds to try to remove troublemakers.
Tear gas and water cannon were used to disperse the protesters, and by late evening just a small group remained.
A nationwide strike has also caused travel chaos throughout the country.
The BBC's Jon Sopel in Paris said the protest had been initially mainly peaceful, but the mood had then deteriorated.
As the trouble erupted in the Place de la Republique in north-east Paris, our correspondent said his BBC crew had also come under attack, although he added the violence had remained "low-level".
VIEWS ON THE STREETS
Opponents and supporters of the CPE air their views
The BBC's Emma Jane Kirby, also in the French capital, said that despite the trouble, the majority of protesters had been marching peacefully.
Extra riot police were deployed before the march, after a rally in Paris last week led to running battles.
French Interior Minister Nicolas Sarkozy told them to only get tough with those he called delinquents.
"My first instruction is that you protect the demonstrators, especially the youngest ones," he said.
"The second instruction is to arrest as many thugs, that means delinquents, as you can."
Mr Sarkozy himself appeared in the Place de la Republique on Tuesday evening, where he congratulated police for their work during hours of sporadic violence.
Correspondents reported a greater sense of calm there, albeit with hundreds of riot police in attendance.
And by 2130 GMT, the majority of protesters had left the square, with only a small hard core of demonstrators reported to be remaining.
Earlier the police used tear gas and water cannon to try to disperse the troublemakers.
Meanwhile workers and students in more than 100 other towns and cities were calling for the government to scrap the controversial employment contracts.
Police estimated that just over one million people had taken to the streets across the country.
They made almost 800 arrests around the country - nearly 500 of them in Paris, the National Police Chief Michel Gaudin told The Associated Press.
Unions said up to 250,000 people turned out for a march in Marseille - many more than at the previous worker-student demonstrations on 18 March - although the police put the figure at 28,000.
In the western city of Nantes, police put the figure at 42,000, more than double the 18 March turnout. Le Mans, Rouen and Tours also reported increased crowds.
"We are here for our children. We are very worried about what will happen to them," said Philippe Decrulle, a demonstrator in Paris.
"My son is 23, and he has no job. That is normal in France," he told the Associated Press news agency.
Meanwhile a nationwide strike closed large parts of France's transport networks.
As transport workers joined the strike, commuters were left battling with widespread disruption as large parts of the country's rail, bus and air networks came to a halt.
Airport authorities warned of delays and cancellations and urged passengers to check with their airlines.
Schools, post-offices, banks, government offices and unemployment bureaux are also experiencing serious disruption.
Protesters are bitterly opposed to the First Employment Contract (CPE), which allows employers to end job contracts for under-26s at any time during a two-year trial period without having to offer an explanation.
The government says it will encourage employers to hire young people but students fear it will erode job stability in a country where more than 20% of 18-to-25-year-olds are unemployed - more than twice the national average.
The BBC's Caroline Wyatt in Paris says these latest demonstrations are a real test of the Prime Minister Dominique de Villepin's resolve, and it is hard to see how he can break the stalemate.
To find a solution without losing face - or ground to his main rival for the presidency, Mr Sarkozy - is proving the biggest challenge of Mr de Villepin's political career, our correspondent adds.
Mr Sarkozy's ruling UMP parliamentary group on Tuesday backed his proposal that the government not rush to enforce the labour laws, instead leaving the door open for further negotiations.
Earlier on Tuesday Mr de Villepin told parliament he was open to talks on employment and possible changes to the contracts, but he did not say he would withdraw them.