The government of Nicolas Sarkozy plans to extend the number of years employees must work to get a full pension to 41 from 40, and to give companies more scope to bypass the 35-hour week.
The 35-hour week was introduced 10 years ago when a socialist government was in power and has been denounced by President Sarkozy as an economic disaster.
Francois Chereque, the head of CFDT, condemned the proposed changes, saying: "Employers will be able to do almost anything they like unilaterally ... The government is opening the way for total deregulation."
The MEDEF, an umbrella body for France's industry bosses, defended the government, but said it could have avoided conflict by heeding an April deal between MEDEF and the main unions on the subject rather than going beyond what they had agreed.
"We would have preferred a different method to move things along ... (but) we consider that the government's proposal is good for companies," Laurence Parisot, the head of MEDEF, said.
The government proposal on the working week would open the way for negotiations at company, rather than industry, level on maximum numbers of overtime hours and compensation for them.
Unions say this would give greater power to bosses, who tend to be stronger in negotiations within individual companies than at industry level where they face the full weight of the unions.