The strike has caused widespread disruptions in the French capital.
It is the biggest test yet for Nicolas Sarkozy, the French president, who was elected on assurances to discard the special pension regime, which allow some state sector employees to retire after 37.5 years of work against 40 years for everyone else.
He has indicated he is willing to make secondary concessions, but has vowed not to go back on his word, confident that public opinion is firmly behind him.
CGT workers at the Paris transport authority (RATP) also voted to stay away from work.
"So we imagine that Friday will go much the same way as today," Jacques Eliez, the union spokesman, said.
Many workers struggled to reach their offices for a second day running and traffic jams clogged roads leading into Paris from well before dawn.
The government wrote to union leaders on Wednesday proposing a month for negotiations between the workers, the companies concerned and state representatives.
"This strike must stop. It's a strike that penalises users and that will not result in social progress," Francois Fillon, France's prime minister, told the senate upper house of parliament.