Train, subway and bus workers, as well as staff at state-run gas and electricity companies are also taking part in the action.
The strike, which is the greatest challenge for the French president since he came to power some six months ago, is expected to severely disrupt the country's national rail network and public transport.
French railway operators said only 90 out of 700 high-speed city trains will be running on Wednesday.
In Paris, all public transport - metro, trams, buses and suburban trains - have been affected, with more than 300 kilometers of traffic jams reported.
Transport trade unions have been angered by proposals made by Sarkozy in September to scrap pension programs for some public sector workers.
The current regime, introduced after World War II, allows some transport workers to retire at 50, and to pay pension contributions for 37.5 years rather than 40, required by other employees.
The government wants to increase the retirement age initially from 50-55 and eventually up to 60. The reform is expected to save some $7.3 billion per year.
Meanwhile, transport workers claim they are low-paid and work longer than other sectors, which justifies their special status.
According to a survey published by L'Express magazine, a total of 58% of respondents are against the strike, with 34% supporting the strikers, and 69% believing the government will not back down.
Several hours before the strike began, Sarkozy told the European Parliament in Strasbourg: "I will carry out these reforms right to the end. Nothing will put me off my goal."
