Unions are up in arms over President Emmanuel Macron's bid to fuse France's 42 different retirement schemes into a single points-based system.
Public transport workers walked off the job for a month and a half in December and January, causing travel misery for millions, particularly in the Paris area.
The government argues that the changes are necessary to make the system fairer for all, but critics say they will force most French people to work longer for smaller payouts.
Labour leaders called for renewed strike action on Monday to coincide with the start of two weeks of parliamentary debates.
But there were only minor disruptions to the Paris metro and regional trains were running as normal.
The start of the debate comes as Macron's centrist party reels from a sex scandal that toppled its candidate for mayor of Paris in next month's municipal election, Benjamin Griveaux.
Griveaux, a close Macron ally and former government spokesman, pulled out of the running over a leaked video showing a man presented as the 42-year-old politician masturbating.
To replace him, the government chose Health Minister Agnes Buzyn, one of the most prominent defenders of the pension reform, which will now be steered through parliament by her successor Olivier Veran.
41,000 amendments
With the government refusing to back down on one of Macron's signature reforms, opposition MPs have stepped into the fray, hoping to derail the bill by tabling a record 41,000 amendments.
Macron's centrist Republic on the Move (LREM) party aims to get the bill through parliament before next month's municipal elections.
While the party has a comfortable majority in parliament, some LREM lawmakers have suggested the legislation may have to be forced through by executive decree if the opposition tries to hold it up indefinitely.
But to do so would see the government accused of curtailing democratic debate on one of the most contentious issues of Macron's presidency.
The reforms would sweep away the separate pension schemes, some dating back hundreds of years, that offer early retirement and other benefits to public-sector workers as well as lawyers, physiotherapists and even Paris Opera employees.
Tens of thousands of people, including large numbers of teachers and doctors, took part in seven separate days of nationwide protests in December and in January.
Retire later
The government argues that the French, who retire earlier than most Europeans, need to work for longer to keep the pension system afloat at a time when people are living longer.
Opponents say the plan will force people to invest in private US-style pension plans and have accused Macron, a former investment banker, of rolling out a red carpet to foreign asset managers like US investment giant BlackRock.
Last week, dozens of activists stormed BlackRock's Paris offices and spray painted the walls and carpets with anticapitalist and environmental slogans -- the second such incident at the site since the pensions battle began.
BlackRock has denied trying to influence the reforms.
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