French President Emmanuel Macron on Tuesday faced opposition alliances crystallising on left and right, after the centrist called a snap parliamentary election in the wake of a European poll defeat at the hands of the far right.

His office delayed until Wednesday a major press conference initially slated for Tuesday afternoon, while insisting that the nationwide vote will put a choice before the French people of "Republican forces on one side and extremist forces on the other".

The forthcoming ballot has set alarm bells ringing across Europe, as it risks hobbling France -- historically a key player in brokering compromise in Brussels and support for Ukraine against Russian invasion.

Fractious left-wing parties moved quickly to set aside differences and form a broad electoral pact, saying they would field joint candidates.

Meanwhile there were rumblings that the formerly dominant mainstream conservative Republicans party would strike a deal with the far-right National Rally, led by Macron's two-time presidential challenger Marine Le Pen.

With just 19 days until the first round on June 30 -- the shortest campaign since France's Fifth Republic was founded in 1958 -- Macron's task to shore up support for his centrist camp is formidable, according to polls.

A Harris Interactive-Toluna poll published on Monday suggested just 19 percent of people would back him, compared to 34 percent for the far-right National Rally.

Macron's prime minister, Gabriel Attal -- who reportedly warned against calling the election -- told party MPs he would "do everything to avoid the worst", his office said.

This election "has more dramatic and historic stakes than that of 2022" because "the extreme right is at the gates of power", he added.

Finance Minister Bruno Le Maire said the choice was "a clear majority or ... the risk of a regime crisis".

On the economic front, ratings agency Moody's warned on Monday that the election posed a risk to its evaluation of France's debt pile of more than three trillion euros ($3.2 trillion) -- around 110 percent of GDP.

Broad right alliance?

Macron called the snap polls after the far right crushed his centrist alliance in Sunday's European Parliament elections.

Analysts said the French leader's move was a bid to ensure the RN did not win the powerful post of president when his second term as head of state ends in 2027.

Le Pen was quick out of the gate with a TV interview late on Monday, calling the vote a "historic chance for the nationalist camp to put France back on track".

She said her policy priorities were "defending purchasing power and fighting insecurity and immigration" and named her 28-year-old party chief Jordan Bardella as prospective prime minister.

Meanwhile Republicans leader Eric Ciotti said he was "seriously thinking about" striking an alliance with the RN, a senior party figure told conservative daily Le Figaro ahead of a lunchtime TV interview.

Once the vehicle that brought presidents Nicolas Sarkozy and Jacques Chirac to power, the Republicains party has in recent years been squeezed between Macron's centrists and the anti-immigrant, anti-EU RN.

The RN is also speaking to smaller far-right outfit Reconquest, which was led into the European elections by Le Pen's niece Marion Marechal.

United left

France's fractious left-wing parties appeared to quickly set aside differences that had shattered their parliamentary alliance, notably their conflicting responses to the war in Gaza.

Socialists, Greens, Communists and the hard-left France Unbowed (LFI) said they would "support joint candidates, right from the first round" of the election -- the same strategy that gleaned them a total 151 seats in the 577-seat parliament in June 2022.

"We did it. We managed to reach a deal," Greens chief Marine Tondelier told activists demonstrating outside the party leaders' gathering at her Paris headquarters.

The left plans to join demonstrations against the far right planned for this weekend by major trade union federations including the CFDT and CGT.

But the alliance has yet to name a consensus candidate for prime minister if it wins most seats.

Party chiefs are reluctant to elevate LFI's abrasive head Jean-Luc Melenchon, while the leader of the Socialist list in the European elections Raphael Glucksmann has counted himself out.

Whatever the decision about leadership, parties across the spectrum are in an organisational scramble to get their candidates' names to election authorities by 6:00 pm on Sunday, before the official start of campaigning next week.