The law has sparked months of protests and anti-government violence and has split President Francois Hollande's Socialist Party, but after some watering down, the government is set on pushing it through.
Prime Minister Manuel Valls was expected to announce within hours that he would bypass opposition in his own political camp to impose a loosening of France's protective labor regulations by decree.
Thousands gathered for what unions acknowledged would be the last street marches before the summer holiday period.
The labor reform, aimed at cutting a 10 percent jobless rate, would also give employers more freedom to tailor pay and job conditions rather than importing ready made rules established via collective bargaining at sector level.
Opponents say that the bill will unravel regulations that have ensured some of Europe's highest standards of labor protection for French workers over the decades.
"This is a counterproductive law socially and economically," said Marie-Jose Kotlicki, a member of the large CGT union, one of several labor and student organizations behind regular protests over the four months since the reform was unveiled.
"The government is making a mistake in underestimating the level of discontent over this law," she said.
Thousands of police turned out in Paris to counter the risk that gangs of youths will engage in ultra-violent battles with police as they during many marches in recent weeks.
Such violence on past protest days have resulted in almost 2,000 arrests and left hundreds of police injured.
Christian Paul, leading a dissident Socialists, warned Prime Minister Valls that he risked further alienating left-wing voters if he overrode parliamentary opponents and forced the labor reform bill onto the statute books by decree ahead of legislative and presidential elections in mid-2017.
"It would be politically devastating," Paul told the iTELE news TV channel. He urged the government to add a guarantee in the bill that overtime pay rates can never go below 25 percent.
"I am telling the prime minister there's a way out. Otherwise things will run off the rails, for the government.
Valls invoked a constitutional clause to force the bill through the National Assembly without a vote during a first review in May and was expected to do likewise this time.