Early voting began in Portugal on Sunday for a snap general election, with polls showing the incumbent Socialists still ahead though with their lead over the nearest centre-right rivals narrowing.

Over 300,000 voters have signed up to vote a week early, with polls opening at 8:00 am (0800 GMT).

The early voting option was provided this year to reduce crowding on election day, next Sunday, due to the pandemic.

"I preferred to vote today as today I'm fine -- next Sunday, I don't know," proffered one early voter, Elisa Fialho, a woman in her 70s who turned out to cast her ballot with her husband in the capital Lisbon.

Prime Minister Antonio Costa, who has led two consecutive minority Socialist governments since 2015, was expected to cast his ballot in the northern city of Porto at around 0930 GMT.

An early voting option has been provided this year to reduce crowding on election day due to Covid-19 An early voting option has been provided this year to reduce crowding on election day due to Covid-19 Photo: AFP / PATRICIA DE MELO MOREIRA

"All the conditions are there for everybody to go out and vote in security," said Costa after voting.

The snap polls were called after Costa failed to secure support for his 2022 budget from the two smaller far-left parties that had been propping up his government.

It was the first time a budget was voted down since the EU member returned to democracy in 1974.

The Socialists have around 38 percent support, compared with just over 30 percent for the main opposition centre-right PSD, which would leave Costa once again without a working majority in parliament, according to a poll of polls compiled by private Radio Renascenca.

But surveys carried out in recent days have shown the gap narrowing, with at least one putting the two parties in a statistical tie when the margin of error is taken into account.

Prime Minister Antonio Costa called the early polls after failing to secure support for his 2022 budget Prime Minister Antonio Costa called the early polls after failing to secure support for his 2022 budget Photo: AFP / Aris Oikonomou

The election could see gains for the upstart far-right party Chega, which entered parliament for the first time with a single seat during the last election in 2019.

Polls give it nearly seven percent support, which would make it the third-biggest force in parliament.

Costa has blasted his former allies -- the Left Bloc and the Communist Party -- as "irresponsible" for voting down his budget and has appealed for voters to give him an outright majority in the 230-seat parliament.

PSD's leader, former Porto mayor Rui Rio, has faced three leadership challenges over the past four years PSD's leader, former Porto mayor Rui Rio, has faced three leadership challenges over the past four years Photo: AFP / PATRICIA DE MELO MOREIRA

During the last election in October 2019 the socialists got 108 seats, eight shy of an outright majority.

"For the first time, I believe" it is possible, Costa said during a recent interview with weekly newspaper Expresso.

If his party wins the most votes but again lacks a majority, Costa has said he plans to govern alone by negotiating support from other parties for laws on a case-by-case basis.

But Lisbon University political scientist Jose Santana Pereira said it will be "difficult" for Costa to form a stable government without the far-left.

"It is foreseeable that the current balance of power will be maintained," he told AFP.

Aiding Costa is the fact that the PSD is divided.

Its leader, former Porto mayor Rui Rio, has faced three leadership challenges over the past four years.

"Costa is a born politician, and in the eyes of voters he is better prepared than Rui Rio," said University of Lisbon political scientist Marina Costa Lobo.

During Costa's first term, Portugal enjoyed four years of economic growth, which allowed the government to reverse austerity measures imposed during Europe's 2011 debt crisis, while posting the country's first budget surplus since 1974.

The last two years have been marked by the Covid-19 health crisis.

Portugal, a nation of around 10 million people, hopes it will soon turn the page on the pandemic thanks to the success of its vaccination programme, which has given it one of the world's highest immunisation rates.

However, like other European nations, Portugal is grappling with a surge in infections fuelled by the highly contagious Omicron variant.

Portuguese voters who are infected and in isolation will be allowed to leave home to cast their ballot on January 30, with a recommended hour-long slot from 6:00-7:00 pm when polling stations are traditionally less busy.

Officials estimated as many as 600,000 people are currently in quarantine.