Thousands of demonstrators heeded a call by the hard-left leader Jean-Luc Mélenchon to demonstrate their opposition to the government’s labour law reforms.

The leader of La France Insoumise (Unbowed France) claimed that 150,000 people were at Place de la République to hear him say: “You are the pride and honour of a country that will not stand and be insulted … it was the people who beat the kings, the Nazis … We are here to defend the republic.”

Mélenchon had called for a show of strength from “la rue” (the streets), the traditional place for French workers to flex their collective muscle. Protesters gathered earlier outside the Paris Opéra house at Bastille to march to the symbolic Place de la République to show their anger at what Mélenchon has called a “social coup d’état”.

“Our country has the record number of millionaires in Europe … Macron is great news for the rich,” said Mélenchon. He urged the crowd: “Put things into motion … young people, enter the fight because you are the first victims of these decrees. The battle is not finished. It has begun. We have to use all the people’s force in the battle and in the streets. Never give in.”

Unions and workers are fighting the changes to the French Code du Travail that they say would make their position more precarious and give bosses greater flexibility to hire and fire, and to express outrage that the reforms have been passed by decree, a little-used political device to avoid a parliamentary vote. Macron signed five decrees changing labour laws on Thursday that were officially published on Saturday and take immediate effect.

La France Insoumise (FI) has just 17 MPs sitting in the 577-seat Assemblée Nationale but has emerged as the opposition to Macron’s administration. The conservative Les Républicains and the Parti Socialiste are both in turmoil following disastrous presidential and parliamentary elections, which gave Macron’s La République en Marche party a clear majority in the lower house of parliament.

The Front National, whose leader Marine Le Pen was in the presidential run-off with Macron, is struggling after the resignation last week of one of the party’s most influential figures, the vice-president Florian Philippot, who has been credited with her recent electoral success.

FI organised 130 buses to bring protesters into Paris for the demonstration. Mélenchon accused Macron and his government last week of provoking a clash. “Democracy is elections, union actions, demonstrations in the streets, marches … all of this. An election isn’t a blank cheque given to those in power,” Mélenchon said.