The various unions behind Tuesday's strike put the number of people taking part in street rallies between 2.5 and 3 million, which they hailed as the biggest turn-out in recent years.
Protests on the same issue in June brought 2 million people out onto the streets.
The Interior Ministry put Tuesday's turn-out considerably lower, however, at just 1.12 million.
The CGT trade union confederation counted 213 protests across France. The largest took place in Paris, where the CGT says 270,000 people attended. Another 200,000 marched in Marseille, and between 40,000 and 100,000 in Bordeaux.
Participation in the national strike that accompanied the rallies was also high, according to official estimates.
Some 24.8 per cent of civil servants walked out, as well as 29.3 per cent of teachers and 42.9 per cent of transport workers.
Some private-sector employees also joined in the strike, including several hundred steel workers and air traffic controllers.
If the government fails to respond to protesters' concerns, "there will be a follow-up and nothing is ruled out at this stage," said Bernard Thibault, head of the CGT.
But ministers defended the pension reforms in parliament on Tuesday.
"By proposing [a minimum retirement age of] 62, the government has made a choice that is both reasonable and essential to cover the cost of France's pensions," Prime Minister François Fillon told MPs.
He said that he was "open to debate so long as we don't lose sight of the objective of reform".
Unions are due to hold a meeting on Wednesday, following which more protests will most likely be called "before the end of the month", according to the head of the CFDT union, François Chérèque.
The majority of the French public supports the protests, according to a poll published by Ifop on Tuesday.
While 53 per cent of those surveyed said the government's proposed reforms were courageous, only 33 per cent thought they were fair, and 70 per cent said the protests were justified.