After violent clashes between riot police and masked youths during previous demonstrations, 130 would-be troublemakers were banned from central Paris to limit the risk of more skirmishes, according to Paris Police Prefect Michel Cadot.
The CGT labor union said the Paris march would be the biggest show of strength since protests over the planned labor reform began in early March.
"This is not the end," CGT leader Philippe Martinez said. "The struggle is far from over."
The CGT, backed by smaller unions in a campaign of strikes and protests, is sparring for pole position with another big union that backs the reform that would also devolve setting of pay and working conditions more extensively to company level.
About 700 buses ferried protesters to the capital from all over France for the march, he said. Smaller protests were being staged in other cities.
Police planning was based on the possibility that more than 50,000 - twice previous levels - would take part, Cadot told a pre-rally news conference.
The risk of more trouble from masked and hooded rioters could not be ruled out, he said.
In tandem, workers stopped work at the state-owned SNCF rail company, which nevertheless said disruption was far less than at the outset of a rolling strike two weeks ago or on previous occasions this year.
Ninety percent of high-speed connections were operating and other services were working at about 70 percent, the SNCF said.
Taxi drivers who are waging a campaign against unregulated competitors protested too, in a demonstration that snarled up traffic on the western edge of the capital.
The CGT union and smaller Force Ouvriere union argue that the reform will undermine standards of labor protection.
The government, and the large CFDT union argue the contrary, saying it will help tackle a jobless rate of 10 percent and also develop labor representation at grassroots level. Youth unemployment is about 24 percent.
"It`s time to calm things down a bit," the head of the pro-reform CFDT union, Laurent Berger, said.
Union membership in France is among the lowest in Europe at less than 10 percent of the workforce. Unions however wield considerable influence because the labor conditions they negotiate are applied to everyone working in any given sector.
Hollande`s Socialist government has refused to withdraw the reform. It forced it through the lower house of parliament by decree last month and aims to make it law by July.
Opinion polls have suggested as many as 80 percent of voters are unhappy with it but they also suggest the protest movement no longer enjoys the backing of a majority of the French people.
Tuesday`s march comes at a time when police are struggling to ensure security during the month-long Euro soccer tournament, with France on maximum terror alert since Islamist militants killed 130 people in November.
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