Thousands of left-wing protesters took to the streets of Paris on Sunday 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 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, which organisers said gathered more than 50,000 people. Melenchon denied the protest was aimed at Hollande, saying: \"This is a left-wing demonstration under a left-wing government.\" 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 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 percent 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.\" Hollande\'s government on Friday unveiled a 2013 budget aimed at plugging the 37-billion-euro hole in France\'s public finances through tax rises and spending cuts. 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.