A leftwing alliance has become the biggest force in the French parliament after tactical voting held back the far right, but the shape of the future government remained uncertain after no group won an absolute majority.

The surprise result for the leftwing New Popular Front – which won 182 seats, followed by president Emmanuel Macron’s centrist Together alliance on 163 and the far right in third with 143 seats – showed the strength of tactical voting against Marine Le Pen’s National Rally (RN). The far right and its allies had forged a commanding lead in the first round but were ultimately held back by massive tactical voting to prevent them winning enough seats to form a government.

Although the left alliance won the most seats, it was more than 100 seats short of an absolute majority. Amid a high turnout estimated at about 67%, no single group won an absolute majority of 289 seats and the ability to form a government. The parliament was likely to be divided into three blocs: the left, centrists and the far right.

France now enters a period of unprecedented uncertainty over the shape of its future government and its likely prime minister. Macron has promised to remain as president, but he did not speak publicly on Sunday night, privately calling after exit polls were released for people to be “prudent” until the final results were clear on Monday morning.

It could now take weeks to establish a government and it was unclear what shape that government could take with the Olympic Games due to begin in Paris in less than three weeks.

The prime minister, Gabriel Attal, announced that he would hand his resignation to president Macron on Monday morning. But he also said he could stay in place for the short term, if required, while a new government was formed.

“Tonight, a new era begins,” he said, adding that France’s destiny would play out “more than ever in parliament”.

Attal said: “I know that, in the light of tonight’s results, a lot of French people feel uncertainty about the future because no majority has emerged. Our country is in an unprecedented political situation and is preparing to welcome the world [at the Olympics] in a few weeks. I will stay in my role as long as duty requires.”

Jockeying for position in the new parliament began instantly. Jean-Luc Mélenchon, leader of the leftwing La France Insoumise party, said: “The president must invite the New Popular Front [left alliance] to govern.” The outgoing interior minister, Gérald Darmanin, said: “I note that today, no one can say they have won this legislative election, especially not Mr Mélenchon.”

Raphaël Glucksmann of Place Publique and the Socialist party, part of the left alliance, said: “We’re ahead, but we’re in a divided parliament … so we’re going to have to act like grownups. We’re going to have to talk, to discuss, to engage in dialogue.”

Despite placing third, the results were historic for the RN – representing its biggest ever score in a parliamentary election, and an increase from the 88 seats it had when parliament was dissolved last month. But it was much lower than the party had expected after it topped the vote in the first round last week.

Jordan Bardella, the RN president, said the parties who had teamed up to stop the far right were a “disgraceful alliance”. Le Pen, who intends to run for president for the far right in 2027, said the far right’s rise to power would continue. She said: “The tide is rising. It did not rise high enough this time, but it continues to rise and our victory has simply been deferred.”

The RN’s limited score showed the success of a tactical voting pact formed last week by centrists and the left to hold back the far right.

More than 200 candidates from the left and centre had pulled out of the second round last week in order to avoid splitting the vote against the RN. Those parties had called on voters to choose any candidate against the RN, in an attempt to prevent the far right winning an absolute majority and forming a government.

The party, which was founded as the Front National by Jean-Marie Le Pen in 1972, was presented by the left and centrists as a danger to democracy that promoted racist, antisemitic and anti-Muslim views. Brice Tinturier, director general of Ipsos, said the results showed that a majority of French voters still saw the RN as dangerous.

Clémence Guetté, who was re-elected for the leftwing La France Insoumise, said the lower-than-expected score for the RN showed that “this is not a racist country and France does not want to be divided”.

On Sunday, Spanish prime minister Pedro Sanchez hailed France’s “rejection of the far right”.

He welcomed the shock result alongside this week’s UK general election where the centre-left Labour party achieved a landslide, saying both countries “have said YES to progress and social progress and NO to going back on rights and freedoms.”

When voting projections were announced there were hugs, screams of joy and tears of relief at the left’s gathering in Paris.

The Place de la République in central Paris was filled with crowds and a party atmosphere, with leftwing supporters playing drums, lighting flares, and chanting “We’ve won! We’ve won!”

“I’m relieved. As a French-Moroccan, a doctor, an ecologist activist, what the far right was proposing to do as a government was craziness,” said 34-year-old Hafsah Hachad.

Macron shocked his own government and party by calling snap elections on 9 June after his centrists were trounced by the far right in European elections.

Angelique Chrisafis is the Guardian's Paris correspondent. Twitter @achrisafis. Click here for Angelique's public key

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