Polls open in one of most important French elections in living memory

Voting has begun in France in one of the country’s most momentous elections in living memory, with the far-right National Rally (RN) of Marine Le Pen and its allies poised to become the dominant force in the national assembly.

After a rest day with no political activity allowed, voting got under way across mainland France at 8am local time on Sunday, with pollsters due to publish usually reliable seat projections as the last big-city polling stations close at 8pm.

Successive polls over the past week have shown the estimated number of RN deputies in the new parliament falling steadily from earlier projections as rival candidates pulled out of three-way run-offs to avoid splitting the anti-far-right vote.

The “republican front” is thought likely to block the anti-immigration, Eurosceptic party from winning an outright majority of 289, with a final Ipsos poll indicating the RN and its allies would send between 175 and 205 deputies to the 577-seat parliament.

Related: ‘We want our peace’: why is France’s far-right support such a rural affair?

That would still give the RN a parliamentary party more than double the size of its 88-strong group in the outgoing parliament, however, with the number of MPs from the centrist coalition of the president, Emmanuel Macron, forecast to halve to at most 148.

The New Popular Front (NFP), a four-way left-wing alliance dominated by the radical left Unbowed France (LFI) of Jean-Luc Mélenchon, was expected to win between 145 and 175 seats, likely making it the second largest force in the new assembly.

Macron, who called the snap election less than a month ago after his camp suffered a humiliating defeat at the hands of the RN in the European parliamentary ballot, looks set to face the final three years of his presidency with no clear ruling majority.

“Today the danger is a majority dominated by the extreme right and that would be catastrophic,” the prime minister, Gabriel Attal, who may be tasked with trying to hold together a caretaker government, said in a final interview on Friday.

Attal promised to stay on “as long as necessary” in a caretaker role while Macron and his advisers plot their next move, which could be some form of broad coalition excluding the far right and far left or perhaps a technocratic government.

But the result risks plunging France – one of the EU’s driving forces and the bloc’s second largest economy, as well as a prominent Nato power and member of the UN security council – into prolonged parliamentary deadlock and political uncertainty.

Le Pen insisted this week the far-right party could win an absolute majority, calling on voters to avoid “a total quagmire” by giving the RN a clear mandate to govern and ensuring her 28-year-old lieutenant, Jordan Bardella, became prime minister.

Le Pen has denounced the republican front as an attempt to steal victory “against the will of the people” by creating a “single party” protecting the political class, and Bardella has said he will not take up the post unless his party has an outright majority.

The RN has toned down many of its positions, but still plans to slash immigration, bar dual nationals from certain state jobs, abolish the right of babies born in France to be French and create a “national preference” for some welfare benefits.

Analysts say the far-right party has benefited from public anger at Macron, whose pro-business reforms have spurred the economy but who is viewed by many voters as having ignored their concerns about the cost of living and worsening public services.

The campaign has been marked by rising tensions and multiple incidents of violence, with more than 50 candidates and campaign activists physically assaulted . Several have been injured to the extent of needing hospital treatment.

The outgoing interior minister, Gérald Darmanin, last week said more than 30 arrests had been made across France and denounced what he described as “a climate of great violence towards politics and all that it represents”.

A fortnight before France is due to host the summer Olympics, about 30,000 police, including 5,000 in Paris, will be deployed this weekend to head off possible post-vote trouble, and street protests have been banned outside parliament.