Chanting "resistance, resistance", the crowds had been rallied by around 60 organisations, including the leftwing Front de Gauche and the French Communist party, which oppose the European budget treaty.
"Today is the day the French people launch a movement against the politics of austerity," said the Front de Gauche president, Jean-Luc Mélenchon.
A few hours before the protest started Jérome Cahuzac, a junior budget minister, described the demonstration as a "fundamental" error. "I think they are committing a fundamental error in thinking that the policies we are following are weakening France, when in fact these policies are strengthening it," he told Europe 1.
The French prime minister, Jean-Marc Ayrault, defended the European budget treaty and accused the protesters of taking a risk with history. "To take the risk of aggravating the crisis, which is not only an economic crisis but also a euro crisis … The ambiguity of saying 'non' is also something that could lead to the end of the euro."
He added that he and the president, François Hollande, "would never be responsible … for the disappearance of the euro. The support of the majority in these circumstances is essential. We can't swerve away, the future of the euro as well as growth and prosperity are in doubt," Ayrault added.
For Annick Coupé of the Solidaires union, the demonstration on Sunday was aimed at creating a "show of force for the weeks to come" in which the government will consider pension, social security and employment reforms.
"Just because we helped defeat Nicolas Sarkozy [the former right of centre president] doesn't mean we're now going to shut up," she said.
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image: © A. Bouirabdane