Union leaders aimed to put 2 million or more protesters onto the streets in nationwide rallies through the day. Stoppages that disrupted rail and air traffic form early morning were expected to hit schools and hospitals.
Francois Chereque, leader of the large CFDT labor union, told RTL radio the government would be ill-advised to ignore what he expected to be the "biggest turnout in a decade."
"After today it's in the government's hands, It they want things to get better they'd better come up with proposals (for changes to the reform)," he said, adding that a general strike was not to be ruled out if the situation deteriorated.
Analysts said however that even if the unions achieved a massive turnout on Tuesday, it was unlikely they could turn the day of protest into a long-term strike movement capable of forcing the government to back down.
Opinion polls show two-thirds of voters think Sarkozy's plan to raise the retirement age to 62 from 60 and make people work longer for a pension is unfair and support the day of protest, but two-thirds also think the strikes will make no difference.
"Never in polling history have the French people been so convinced that there's a social injustice," said political analyst Roland Cayrol, from Paris's Sciences Po university.
The conservative government says the reform is essential to balance pension accounts by 2018, reduce the public deficit and preserve France's top-notch AAA credit rating, which helps it to service a big debt as cheaply as possible in financial markets.
The strikes cut rail services by 50 percent and more in many cases but did not hit international links. Urban underground train services were also hit, although less than feared in the early stages of Tuesday, according to the RATP Paris urban transport company.
Dozens of street rallies were programmer, starting with many major cities in the morning before an afteroon march in Paris.
The labor unrest mirrors action in several European countries against austerity measures imposed to reduce budget deficits swollen by the 2008-9 economic crisis. Governments from Greece, Spain, Italy and Romania have faced down strikers to impose painful pay and public spending cuts.
On the eve of what may be the biggest protests since Sarkozy won office in 2007, ministers said the pension bill's key principles were non-negotiable but signalled concessions on secondary issues such as earlier retirement for those in physically demanding jobs or who began work at an early age.
Most major European countries have an official retirement age of 65, and some, such as Germany and Britain, plan to raise it gradually to 67 or beyond. But the real effective retirement age in France is similar to that of its neighbors, according to the Organization for Economic Cooperation and Development.
Martine Aubry, head of the opposition Socialists, accused the government of misleading the people about conditions in other countries. She said Germans could secure a full pension after fewer years of work than the French and that her party would, if in power, revert to a legal retirement age of 60.
With Sarkozy's approval ratings close to all-time lows, the president's two top advisers hinted on Sunday he may amend a widely-criticized "tax shield," enacted to ensure the wealthy do not pay more than 50 percent of their income to the state.
While the protesters march, Labour Minister Eric Woerth, battered by a scandal over alleged conflicts of interest and illegal political donations, will introduce the pension reform in parliament, which is due to adopt the bill next month.
Sarkozy has stood by Woerth despite a string of disclosures about his links with France's richest woman, L'Oreal heiress Liliane Bettencourt, and her wealth manager, Patrice de Maistre. The minister has denied wrongdoing and refused to resign.
However, the scandal has weakened his position and helped drive protests against the pension bill, seen as Sarkozy's flagship reform and the springboard for his bid for re-election in 2012.
France has a tradition of militancy although only about 10 percent of workers, mostly in the public sector, are unionized. A 1995 revolt forced former President Jacques Chirac to abandon planned pension and healthcare reforms, and 2006 student protests killed plans for a low-wage youth employment contract.
But the mood is different now, with many people accepting the notion that longer life expectancy and weaker public finances make a raising of the retirement age inevitable.
"Unlike 1995, when similar numbers took to the streets, there isn't the feeling (this time) that they can win somthing out of it," said Sciences Po analyst Cayrol.
