The reforms, which were significantly watered down under pressure from a wave of protests, were adopted earlier Thursday by the cabinet of an increasingly unpopular President Francois Hollande, who hopes to stand for re-election next year.
"Young and insurgent, the world is ours" read one banner as hundreds gathered at Place d'Italie in the south of the French capital, where riot police used tear gas after students threw bottles and emptied a rubbish bin over some officers.
A protest in Nantes also turned violent as students hurled bottles at security forces who used tear gas and truncheons against the youths, making nine arrests, police said.
Students have been at the forefront of protests over the reforms aimed at freeing up the job market and reining in France's 10 percent unemployment rate.
Among youths, joblessness is nearer to 25 percent – among the highest in Europe.
The youths, along with unions and the left flank of Hollande's Socialist Party, say the reforms are too pro-business and threaten hallowed workers' rights.
One of Thursday's protesters in Paris was dressed as a capitalist, wearing a top hat and smoking a cigar with a sign reading "Business, power, finance – all together!"
Some 58 percent of the French people oppose the measures, according to a recent poll.
Prime Minister Manuel Valls has touted the reforms as "intelligent, audacious and necessary", in particular for reversing unemployment, which has not dropped below seven percent in 30 years.
"Our country has become used to (joblessness) for too long," he said.
The pressure prompted the government this month to water down the contested reforms, walking a tightrope between the insistent demands of employers and employees.
Employers call for reform
Bosses were unhappy with the withdrawal of a cap on the amount companies must pay for unfair dismissal, as well as the scrapping of a measure that would have allowed small and medium-sized companies to unilaterally introduce flexible working hours.
While employers' groups called on Valls to restore the reforms' original goal of creating jobs, the concessions were enough to persuade several so-called "reformist" unions to get behind the new version while still urging new language on conditions for laying off workers.
The seven unions and youth groups which called Thursday's protests are demanding the withdrawal of the reforms.
Last week students paralysed dozens of schools and universities across France, and on Thursday students were again barricading campus entrances in Paris.
Tense protests were also under way in western Rennes and southern Marseille.
Socialist Party dissidents, threatening stiff resistance when the reforms reach parliament, have presented a "counter-reform".
The government's proposed reforms are scheduled to be taken up by parliament's social affairs committee on April 5, and by the full body in late April or early May.
But before that, the protest movement plans an even bigger mobilisation for March 31.