French President Emmanuel Macron's controversial pension reform was introduced to the country's parliament, the Assemblée Nationale, on Monday.

The debates around the project are expected to be heated after months of strikes and street protests against the changes, which will overhaul France's expensive and complex pension system.

It comes at a difficult time for the parliamentary majority of Macron's La Republique en Marche (LREM), which struggled with a chaotic episode last week when the party's Paris mayoral candidate Benjamin Griveaux had to resign following the publication of a sex tape.

The former Health minister Agnes Buzyn replaced Griveaux in the race, leading to the nomination of Olivier Véran, previously one of the government officials in charge of the pension reform.

MPs from Macron's LREM party will now have to defend the project deemed "unjust" by much of the opposition, which has filed 41,000 amendments against it.

President Macron has asked his parliamentary majority for "unity" to "win the battle of the pensions" and "sell" the highly controversial law, aimed at creating a universal point-based pensions system.

Debates on the law are planned for two weeks, with a possible third week, before the parliament takes a break for the French municipal elections on March 15 and 22.

The Assemblee nationale's president Richard Ferrand, from the LREM party, failed on Monday to limit the debates on the pensions reform to 100 hours over three weeks, the AFP reported, after left-wing MPs voted against the limit.

"Why would we make their [LREM] work easier?" left-wing leader Jean-Luc Melenchon said, adding that his party "does not want this law" to pass.

Trade unions have called for renewed protests as the law went to the parliament on Monday.

On the Paris metro, traffic was disrupted on three lines according to the RATP transport system.

Philippe Martinez, the head of the country's main union CGT, criticised Buzyn's departure from the Health ministry, saying that ministers "should see to the end of their briefs".

Socialist leader Olivier Faure has said it is a sign of "this government's frivolity", which is trying to "force [the reform] through".

Both left and right denounce a "nebulous" and "incomplete" project. Right-wing party Les Republicains (LR) has said the government "lacks preparation".

LREM MPs vow to pursue the project "through thick and thin", despite reports of doubts within the parliamentary majority.

An unnamed LREM official quoted by AFP said the project would be "an exercise in strong collective discipline".

A group of 15 Communist MPs arrived at the Assemblee Nationale for the first reading on Monday with t-shirts that read 'Referendum", to call for a public vote on the pensions reform.

Their motion calling for a referendum "has received a sufficient number of signatures to be filed", Communist leader André Chassaigne told AFP. "The French people in its great majority is united behind us", he said.