LISBON (Reuters) -Portugal's far-right, anti-establishment party Chega overtook the centre-left Socialists to become the main opposition party for the first time on Wednesday after the final tally of ballots from abroad in a May 18 snap parliamentary election.

The centre-right Democratic Alliance of Prime Minister Luis Montenegro won 91 seats in the 230-seat single-chamber parliament, including two from out-of-country voting. While it garnered more seats than in 2024, it again fell short of a majority needed to end a long period of instability.

On the election night, the Socialist Party (PS) emerged slightly ahead of Chega in terms of the share of the vote but with the same 58 seats. The final tally, published by the Interior Ministry on Wednesday, took Chega's parliamentary representation to 60 while the PS was left with 58.

Founded just six years ago, Chega, thus ended five decades of dominance by Portugal's two mainstream parties after the end of a fascist dictatorship in 1974, chiming with similar advances for the hard right across Europe.

Chega has allied with anti-immigration parties, such as Marine le Pen's National Rally in France and Germany's AfD.

It has long campaigned accusing the country's political establishment of perpetuating corruption, while also calling for an end to "open doors" immigration and tougher sentences for criminals, including chemical castration for repeat rapists.

Montenegro has refused to make deals with Chega and said he would form a new minority government.

President Marcelo Rebelo de Sousa will meet the leaders of the three main parties on Thursday and is then widely expected to name Montenegro as the prime minister.

(Reporting by Andrei Khalip, edititing by Sergio Goncalves and David Latona)