France's left Friday vowed a "total break" with President Emmanuel Macron's policies if its new alliance wins historic polls that could propel the far right to major gains in parliament.

Far-right figurehead Marine Le Pen, also making a pitch to voters, pledged a "national unity government" if her party takes power in the snap legislative elections.

Macron on Sunday stunned France by calling polls after Le Pen's far-right National Rally (RN) scored more than double his centrist alliance's result in last week's European elections.

Left-wing groups including hard-left France Unbowed (LFI), the Socialist, Communist and Green parties on Thursday agreed an election alliance called the New Popular Front.

On Friday, they unveiled a joint manifesto, whose headline measures included jettisoning Macron's controversial immigration and pension reforms if they win the polls, which open on June 30 with a second round on July 7.

They also promised to "rise to the climate challenge" -- without agreeing on whether to go ahead with modernising France's fleet of nuclear plants -- and to maintain support for Ukraine against the Russian invasion.

"It's going to be either the far right, or us," Greens party leader Marine Tondelier told reporters.

The coalition won backing from leading left-wing politician Raphael Glucksmann, 44, who led the Socialist-backed list in the European elections.

"We can't leave France to the Le Pen family," he told broadcaster France Inter.

The name of the alliance is a nod to the Popular Front, a political alliance founded in France in 1936 to combat fascism.

Opinion polls suggest Le Pen's party will massively increase its parliamentary presence from its current 88 out of 577 seats.

She took over the National Front -- founded in 1972 by a former SS member -- from her father in 2011, renaming it and standing three times as its presidential candidate.

- 'Hate and discrimination' -

Francois Hollande, the Socialist former president, backed the new union, saying the left-wing forces had "got beyond our differences".

It remained unclear however who would lead the New Popular Front and become prime minister in case of victory. Glucksmann ruled out the LFI's abrasive leader Jean-Luc Melenchon.

Aurelien Rousseau, a former health minister under Macron, announced on Friday he was switching his allegiance to the Popular Front.

"The RN must not come to power," he said, adding that only the Popular Front was capable of stopping it.

Hitting the campaign trail in the Pas-de-Calais region of northern France, Le Pen claimed the RN could win the elections and form a "national unity government".

"We need to pull France out of the rut," said the 55-year-old, who is expected to run for a fourth time in the 2027 presidential election.

The country was in a "catastrophic situation", she added.

The far right suffered one setback Friday in the shape of an Instagram post from one of France's top YouTubers, Squeezie -- the alias of 28-year-old Lucas Hauchard.

"I've never wanted to talk to you about politics," he told his almost nine million Instagram followers.

"But I think firmly opposing an extremist ideology that preaches hate and discrimination goes beyond any kind of political positioning," he said. The post garnered almost 900,000 likes within a few hours.

- 'Profoundly wrong' -

Other right-wing forces were mired in infighting.

Eric Ciotti, leader of the conservative Republicans, broke a historic taboo this week, announcing his party would form an electoral alliance with the RN.

The rest of the party leadership promptly expelled him, confirming the decision with a second vote on Friday according to party sources.

But Ciotti appeared to have successfully challenged their decision Friday. A Paris court suspended the decision against him pending a more in-depth ruling within eight days.

The 28-year-old RN chairman, Jordan Bardella, said the far-right party and the Republicans would put up joint candidates in 70 of France's 577 parliamentary constituencies, hailing what he said was a "historic agreement".

Macron remained defiant, defending his decision to dissolve parliament and call snap elections.

Speaking at a G7 summit in southern Italy on Thursday, he said his G7 counterparts had praised his move.

"They all said: 'This is courageous'", Macron told journalists.

He took time too, to take a swipe at the programmes of both the Popular Front and the National Rally, describing them as "totally unrealistic".

Italy's far-right Prime Minister Giorgia Meloni on Thursday accused Macron of seeking to score points with voters at home, saying it was "profoundly wrong" to use the G7 summit for "campaigning".

France's stock market suffered its worst week since March 2022 and the first weeks of Russia's invasion of Ukraine.

The CAC 40 index fell 6.23 percent between the election announcement and close of trading Friday.