CAIRO (AFP/Reuters) - Former army chief Abdel Fattah al-Sisi won 96.9 per cent of votes in Egypt's presidential election, the electoral commission announced on Tuesday, almost a year after he overthrew elected Islamist leader Mohamed Mursi.

Turnout in last week's election, hastily extended to three days amid fears of low turnout, was 47.45 per cent, said commission chief Anwar Rashad al-Asi. It was less than the 40 million votes, or 80 per cent of the electorate, that Mr Sisi had called for. Egypt has 54 million voters.

Mr Sisi's rival Hamdeen Sabbahi won just 3 per cent of the vote, excluding spoiled ballots.

Mr Sisi's lopsided victory had been certain, with many lauding the retired field marshal as a hero for ending Mursi's divise rule in July.

Yet the lower-than-expected turnout - Mr Sisi himself had called for much more voters to come out - signalled that a wide segment of the population was apathetic or boycotted the election.

Mursi's Muslim Brotherhood, crushed by a massive crackdown following his overthrow and detention, had boycotted the vote.

Some journalists and government employees erupted in applause and began dancing as the final results were announced at a press conference on Tuesday.

Mr Sisi now faces a wide array of challenges in a country where street demonstrations have helped to topple two leaders in three years.

The lower-than-expected turnout raises questions about his ability to maintain his popularity while attempting to fix a battered economy, ease poverty and prevent further political crises from paralysing Egypt.

For now, Mr Sisi's supporters seemed content to celebrate the moment, gathering by the thousands in Cairo's Tahrir Square, the symbolic heart of the uprising that ousted autocrat Hosni Mubarak in 2011.

Meanwhile, Saudi Arabia's King Abdullah, hailing Mr Sisi's "historic" win, has invited countries to attend a donor conference for Egypt, his country's most populous Arab ally.

The King, whose country is a strong ally of Egypt, said Mr Sisi's sweeping win represents a "historic day and a new stage for Egypt", in a telegram published by SPA state news agency.

"To the brothers and friends of Egypt... I invite all to a donors conference... to help it overcome its economic crisis," he said.

The world's top oil exporter has been a main supporter of Mr Sisi and Egypt's military government since the army overthrew president Mohamed Mursi of the Muslim Brotherhood last summer. Saudi Arabia, Kuwait and the United Arab Emirates pledged US$12 billion (S$15 billion) in aid then, but have not announced any new commitments.