Unions hoped to draw half a million people into the streets, with six in 10 French people saying they supported the movement, according to a poll in France's Liberation newspaper.
Unions said 150,000 protesters turned out for early protests on Thursday in Nantes, Rouen and Le Havre in the west, Grenoble and Marseille in the southeast and Clermont-Ferrand in the centre of France.
Police said the total was 40,000 demonstrators.
Meanwhile, walkouts were reported across the French public sector, with between 10 and 20 per cent of postal, utilities and telecoms employees joining the strike, officials said.
Morning radio slots were replaced by music on public stations France Inter and Radio France Internationale.
Factory workers and dockers angry at privatisation plans joined the marches in several cities including Marseille and Calais.
France's five main unions are leading protests against plans to increase the number of years worked needed to draw a full pension, from 40 to 41, starting next year.
Written into a reform passed in 2003, the pension changes continue to raise hackles in union ranks.
"There are other financial solutions aside from asking workers to stay at work longer," said Bernard Thibault, head of France's biggest union, the CGT.
"The government is going to have to change its line under pressure."
Francois Chereque, head of the CFDT union, has accepted the reform in principle but says it should take into account the physical strain, and the knock-on effect on life expectancy, of various lines of work.
However the government of Nicolas Sarkozy, France's president, has vowed to stand firm.
Speaking on France 2 television on Wednesday night, Francois Fillon, the country's prime minister, said: "The question was settled in 2003."
Thursday's strike comes hard on the heels of a two-week protest movement by fishermen who have blockaded oil depots, clashed with police and disrupted cross-Channel traffic in a two-week protest over rising fuel prices.
Fishermens' leaders called on Wednesday for an end to the protests after the government offered an aid package of $173m to the workers.