Organizers said more than 50,000 demonstrators and representatives from 60 organizations marched through central Paris to the Place d'Italie on Sunday, chanting "Resistance!"

"This is the day the French people launch a movement against the policy of austerity," former presidential contender and Left Front leader Jean-Luc Mélenchon said at the march.

Fiscal pact debate

The demonstrations come on the eve of Tuesday's debate on the ratification of the fiscal pact.

The pact was agreed by EU leaders back in March, but it requires that the participating nations sign into a law a commitment to limit their structural deficits to within 0.5 percent of their gross domestic product.

Prime Minister Jean-Marc Ayrault will launch a French debate on the pact in the National Assembly on Tuesday.

Speaking to deputies from the left-wing PRG party on Sunday, Ayrault said France must approve the pact as an "essential step" toward resolving the eurozone debt crisis.

"Francois Hollande and I will never take the responsibility... of making the euro disappear," he said, according to the Agence France Presse. "The future of the eurozone is at stake."

Many on the French left, however, have said they will vote against the measure. Despite resistance from far-left parties, the Greens and some dissident Socialists, the pact is expected to be approved because the main conservative party and most of President Francois Hollande's Socialist Party support it.

Sign of more to come

Sunday's demonstrations come just two days after Hollande's government unveiled its 2013 budget, which aims to plug the nearly 37-billion-euro hole in France's finances by raising taxes and cutting spending.

The protests were the first major ones again the new socialist president's policies.

Hollande's campaign had pledged to avoid the overly harsh austerity plans adopted elsewhere in Europe.

Mélenchon said the Sunday protests in Paris were just the start of more to come.

tm/ccp (AFP, dpa, Reuters)