Earlier, the French National Railway Network (SNCF) said only a fraction of trains would run, while bus and tram services faced disruption in 27 cities.
Guillaume Pepy, SNCF's executive director-general, told Le Parisien newspaper: "Traffic will be reduced today with less than five per cent of trains running compared with a normal day."
France will host Saturday's final of the rugby World Cup, with tens of thousands of British and South African fans expected in the capital.
Gas and power utility staff, who would also be subject to the pension rules, were due to walk off the job, although cuts in supplies were not expected.
Airlines could be affected if air traffic controllers were unable to get to work, the authorities said, though most cross-Channel Eurostar trains were due to run as normal.
The government wants the employees to pay into the so-called "special regime" pension schemes on an equal footing with civil servants and private sector workers, increasing their contribution period from 37.5 years to 40 years.
The special funds were introduced after the second world war, mainly for workers in physically demanding jobs, but they are running deficits that will cost the taxpayer an estimated $7bn this year.
Sarkozy said during his election campaign that he would phase out the regimes. He has offered talks on how best to do that while refusing to back down on the principle.
|"Now is the time for the government to listen to the rail workers and withdraw its proposals" |
Stephane Leblanc of the Sud-Rail union
Stephane Leblanc of the Sud-Rail union predicted "an historic mobilisation" during the strike action as SNCF managers were also taking part.
"Now is the time for the government to listen to the rail workers and withdraw its proposals," he said. Other unions have hinted that the protests could extend beyond Thursday.
Lengthy protests have crippled France in the past, proving fatal 12 years ago to a conservative government which was forced into a humiliating climb-down over similar pension changes.
All sides have played down the 1995 parallel. An Ifop poll on Wednesday showed 82 per cent of people backed reform of the "special regimes", and 61 per cent opposed the strike.
The senior members of the Socialist opposition and even unions agree that state pensions must be reformed, but argue that workers should not bear the brunt of any changes.