Unions are hailing the strike as the biggest yet in the long-running battle against the plan. Turnout is slightly higher than last time due to the participation of high school pupils. Many teachers, transport workers and civil servants have joined the action.
The protesters say the reform, which takes the retirement age from 60 to 62, is an unfair burden on workers.
But President Nicolas Sarkozy's government is digging in its heels, saying the changes are necessary to bring down the public deficit.
"We have reached the limit of what is possible" in amending the bill, Prime Minister François Fillon said today.
The government last week did amend the bill to add certain exemptions for working mothers, but unions rejected the concessions as inadequate and pressed on with open-ended strikes.
See here for details of disruptions to transport.