Chanting "Resistance!" protesters marched through central Paris in a rally organisers said was aimed at fighting EU-imposed austerity, not at criticising the government of Socialist President Francois Hollande.
"This day is the day the French people launch a movement against the policy of austerity," one-time presidential contender and Left Front leader Jean-Luc Melenchon said at the march.
Melenchon denied the protest was aimed at Hollande, saying: "This is a left-wing demonstration under a left wing government."
Organisers said 80,000 people took part in the rally. The Paris prefecture said that in keeping with its policy for demonstrations by political parties, it would not release an estimate.
"It was very successful, beyond our expectations," said Annick Coupe, a spokeswoman for the Solidaires union.
Prime Minister Jean-Marc Ayrault is set to open what is expected to be a long and difficult debate on the fiscal pact in the National Assembly on Tuesday.
The pact, agreed by EU leaders in March, requires its signatories to write into law a commitment to limit structural deficits to within 0.5% of gross domestic product under normal circumstances.
Many on the French left — including within the Socialists and their Green Party allies — have said they will vote against the measure, but with right-wing deputies backing the pact, it is expected to be approved.
Speaking to deputies from the Radical Left Party (PRG), Ayrault said approving the pact would be an "essential step" in resolving the debt crisis threatening the eurozone and its single currency.
"Francois Hollande and I will never take the responsibility… of making the euro disappear," he said. "The future of the eurozone is at stake."
Hollande's government unveiled a 2013 budget Friday aimed at plugging the €37 billion ($47 billion) hole in France's public finances through tax increases and spending cuts.
But Francois Fillon, who served as prime minister during Nicolas Sarkozy's presidency, said the march showed how disillusion was taking hold of the country, including a large part of the left.
About 1,500 people also protested against austerity measures in Brussels, police said, in a demonstration called by left-wing groups and unions.
The protest was aimed at pushing for measures in Belgium and Europe "to fight against poverty and share prosperity equally," the CSC union said.
The Paris march came a day after tens of thousands of people massed on the streets of the Portuguese capital Lisbon, and thousands in the streets of Madrid, in neighbouring Spain, to protest austerity cuts.
In Lisbon, protesters rallied against an austerity programme expected to get even tougher to meet pledges made to the country's international creditors.
In Madrid, demonstrators faced off with riot police and denounced the conservative government's deep budget cuts.