Entering the French lower house of parliament as an MP for the first time last week, Jean-Luc Mélenchon pointed to the European flag planted next to the French tricolor, turned to the camera tracking him and said: “Do we have to put up with that?”

Earlier he stood on the steps of the Assemblée Nationale, alongside the other 16 newly elected MPs from his hard-left party La France Insoumise (France Unbowed), raised a clenched fist and shouted “Resistance”.

Mélenchon declared they were there as “opposition MPs in the service of the people”. He had begun as he means to go on for the next five years – going head-to-head with president Emmanuel Macron’s La République en Marche (La REM) majority government.

It is a battle that will be fought in parliament and – as Mélenchon has made it clear – out on the streets if necessary.

Macron, a former investment banker who is deeply pro-Europe, is seeking to loosen France’s complex labour laws to allow companies to hire and fire more easily, negotiate working hours and wages with employees and not the unions, and cap unfair dismissal pay-outs. France’s youngest president is planning to use “ordinances” – a process to push through legislation quickly by decree – which French unions will bitterly contest as “sweeping away social dialogue and consultation”. He has also pledged to cut public spending by €60bn and lose 120,000 public-sector jobs. Mélenchon has promised not a single concession on workers’ rights without a fight.

His party has only 17 seats – out of a total of 577 in the National Assembly – but is at least a unified opposition, which is more than can be said for the general election runners-up, the conservative Républicains, which won 112 constituencies but is currently tearing itself apart, or the Socialist party, which is also catastrophically riven and now has just 29 seats – compared with 295 in 2012. Macron’s REM has 308 seats and his allied Democratic Movement, MoDem, party has 42.

Mélenchon gearing up for the second round of parliamentary elections earlier this month. Photograph: Claude Paris/AP

Even so, the political scientist Dominique Reynié, director of the progressive centre-right thinktank Fondapol, said he doubted everything would go the way Macron wanted once the electoral honeymoon period was over. “He will face opposition. If not in parliament then outside, on the streets,” Reynié said.

Bruno Jeanbart, deputy director of the pollsters OpinionWay, had already warned even before Macron’s triumph: “Where is the opposition? If it doesn’t happen in parliament, it will happen in the streets, in the press.”

Their warnings were echoed this week by Luc Rouban, a political scientist at the Centre National de la Recherche Scientifique, who described the political situation in France as “potentially explosive”. Rouban said FI could put up “a little resistance” but “opposition is likely to express itself outside parliament.”

Even before he took up his seat in the Assemblée Nationale, Mélenchon was making headlines. Referring to one of Macron’s more flamboyant new and inexperienced MPs, the prizewinning mathematician Cédric Villani, as “the maths guy”, he added: “I’ll explain to him what the labour law is all about and he’ll be astonished. He’s no idea what’s in it! He doesn’t realise the eight-hour working day was the result of 100 years of battle.” Villani responded in good humour. “Dear Jean-Luc Mélenchon,” he tweeted. “As director of IHP [a mathematical research centre], I’ve seen work contracts. But it’s always a pleasure to have a private lesson!”

Rouban said that Mélenchon and the far-right Front National leader Marine Le Pen, who was also elected to the French parliament for the first time with seven other FN MPs, could become the cheerleaders for a “social challenge”, a strong theme for the presidential election”. The situation was made even more unpredictable, he said, because the opposition parties had little real power. And the decider would be “whether what they say carries any weight with public opinion or whether there is a form of apathy among the working classes and of patience among the upper classes”.

Pierre Gattaz, head of the French business leaders’ organisation Medef, dismissed the idea of Mélenchon leading any kind of credible opposition to Macron. He said Mélenchon’s worship of Cuba’s Fidel Castro and Venezuela’s Hugo Chávez made him a man whose ideas were “extremely dangerous”.

“He can talk. He has a great talent for oratory, but we’ll have to see how it finishes for those who put their faith in someone who speaks well, but whose ideas will lead to ruin and desolation for France,” Gattaz told the Anglo-American Press Association.

“We have to call a spade a spade. He has never produced a single idea for creating jobs in France. Let Mr Mélenchon set up his own company and create a few jobs and then he can say something.”

Gattaz, who believe’s Macron’s economic reform plans “do not go far enough” said he was “optimistic” that reform would happen. If not, “we will be looking at Mélenchon and Le Pen in the second round in 2022,” he said.

Asked where he saw opposition to Macron’s economically liberal programme coming from, Gattaz said “possibly from the streets”.

The historian Jean Garrigues said opposition parties had few weapons against an absolute parliamentary majority, adding that “the opposition vote against can’t make much difference” and they had the choice of ganging up on the government by joining forces or taking the fight to the streets. The latter only worked “when there’s pressure from unions and public opinion”. Olivier Rozenberg, associate professor at SciencesPo university, said: “The opposition isn’t going to change laws, but they can make their point of view heard. They force the majority to justify itself, which is important.”

Mélenchon believes that his best ally is the record 57% bloc of French voters who, orphaned by the disintegration of the traditional left and right parties, did not bother to cast a vote in the general election. “The president has no legitimacy to perpetrate a social coup. I see in this abstention an energy that’s available if we know how to use it for our fight,” he said.