Thousands of left-wing protesters have taken to the streets of Paris to denounce the European Union fiscal pact forcing governments to stick to tough deficit limits.
Chanting "Resistance!", protesters marched through central Paris in a rally organisers say 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 on Sunday.
Mr Melenchon denied the protest was aimed at Mr Hollande, saying: "This is a left-wing demonstration under a left-wing government."
Organisers said 80,000 people took part in the rally. "It was very successful, beyond our expectations," said Annick Coupe, a spokeswoman for the Solidaires union.
Prime Minister Jean-Marc Ayrault is to open what is expected to be a long and difficult debate on the fiscal pact in the National Assembly tomorrow .
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 per cent 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 left-wing PRG party on Sunday, 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."
Mr Hollande's government on Friday unveiled a 2013 budget aimed at plugging the 37-billion-euro ($A46 billion) hole in France's public finances through tax rises and spending cuts.
About 1500 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 the day after tens of thousands of people massed on the streets of Portugal's and Spain's capitals to protest austerity cuts.
In Madrid, demonstrators faced off with riot police and denounced the conservative government's deep budget cuts, while in Lisbon, protesters rallied against an austerity program that is expected to get even tougher to meet pledges made to the country's international creditors.