Services on two of Europe's most-used rail lines - the RER A and B in the Paris region - were cut by half on Thursday and about 1,000 people rallied on Paris's Place de la Nation, with demonstrations in several other cities, in response to the CGT's call to protest at a "witch-hunt" against trade union activists.
The strike was called as a show of solidarity with eight trade unionists at the now-closed Goodyear tyre plant in Amiens, north-east France, who held their bosses captive for 30 hours in 2014.
Last month a court sentenced them to nine months in prison with a further 15 suspended, an unprecedented sentence for workers involved in a labour dispute.
One of the activists, Mikaël Wamen, called for the case against them to be dropped at the Paris rally.
The union has accused the government of being behind the sentences, a charge denied by Prime Minister Manuel Valls who insisted that the courts are independent.
But the CGT points to several other legal cases linked to labour disputes, notably the forthcoming trial of union members at Air France for their alleged involvement in a protest that saw the shirt ripped off the back of the company's human resources boss.
A petition in support of the Goodyear eight has gathered 150,000 signatures.