Lawmaker Laurent Pietraszewski will oversee the negotiations, replacing the last top official who was forced to resign on Monday when it emerged he had failed to declare income.
Teachers, hospital workers and other public employees joined transport workers on Tuesday for the third big day of marches since the dispute began on December 5.
The interior ministry said about 615,000 people took part in more than 100 rallies countrywide, including 76,000 demonstrators in Paris, where the Eiffel Tower was closed due to the protest.
The hardline CGT union tweeted that 1.8 million demonstrators had turned out across the country, a figure higher than the 1.5 million it claimed for the last big protest day on December 5.
The CGT said electricity workers had cut power to some 50,000 homes near Bordeaux and 40,000 in Lyon overnight, warning that bigger cuts could follow.
Later the CGT and four other trade unions issued a joint ultimatum to the government, saying local industrial action would continue with no let-up for Christmas, unless it responds to their requests "in the coming hours".
The day of action was "a total success", said CGT leader Philippe Martinez. "Despite the government's attempts at division, the people remain mobilised," he added.
The government has insisted it will push through a single points-based pension system and end the current patchwork of 42 separate schemes that offer early retirement to many in the public sector.
It says the new system will be fairer and more transparent, improving pensions for women and low earners in particular.
"My determination, and that of the government and the majority, is total," Prime Minister Edouard Philippe told parliament on the eve of fresh talks with unions.
Critics say the changes could force millions of people to work beyond the official retirement age of 62 -- one of the lowest in Europe -- by setting a "pivot age" of 64 that would ensure a full pension.
"What scares us about the points system is that we don't know how much a point is worth," said Kelly Grosset-Curtet, a 21-year-old student marching in Lyon.
Laurent Berger, head of France's largest union, the moderate CFDT, took part in the Paris protest and described the pension reform measures as "terribly unjust".
Commuters in Paris and other big cities have borne the brunt of the transport stoppages so far but holiday travel plans are now at risk, with just one in four high-speed TGV trains running on Tuesday.
Strike organisers are hoping for a repeat of 1995 when they forced the government to back down on pension reform after three weeks of metro and rail stoppages just before Christmas.
Some 62 percent of respondents to a poll for the RTL broadcaster said they support the strike but 69 percent said they wanted a "Christmas truce".
"Nobody wants to mess up Christmas, not the strikers nor workers nor the French who want to be with their families," Laurent Escure of the UNSA union told France 2 television.
"But this is entirely the government's fault." Train operator SNCF has warned that it may now be too late to get services back to normal by December 25. But it assured that all TGV ticket holders would be able to travel over the pre-Christmas weekend, though with some changes to timetables.
Several universities have cancelled or postponed year-end exams, and both the Garnier and Bastille Operas in Paris have cancelled dozens of performances, costing millions of euros in lost ticket sales.
"This is absurd," Sylvie Baheux, a 55-year-old gym teacher, said at Paris' Saint-Lazare station on Tuesday, adding that her usual one-hour commute to work had doubled during the strike.
"It's complicated but this pension reform needed to be done," she told AFP. Opposition leaders have urged the government to rethink the pension reform plans.
The travel misery is set to continue on the 14th day of the strike Wednesday, with half of the capital's 16 metro lines closed, most of the rest severely curtailed, and the number of regional and suburban trains slashed.