More than 98 percent of voters backed a new Egyptian constitution in a referendum this week, authorities said, though the turnout was lower than some officials had indicated, with under 40 percent of the electorate taking part.

The vote advances a transition plan that army chief General Abdel Fattah Al-Sisi unveiled after deposing Islamist president Mohamed Mursi last July following mass protests against his rule.

Sisi, whom many Egyptians see as the strong leader who can stabilise their country after nearly three years of turmoil, is expected to announce his candidacy for the presidency soon.

"Now that God has supported us in legalising our constitution, we ask for his aid in achieving the remaining two stages of the road map: the presidential and parliamentary elections," said Nabil Salib, head of the Supreme Election Committee.

The "yes" vote was 98.1 percent, and 38.6 percent of eligible voters took part, Salib told reporters.

The turnout was well below the 55 percent that an Interior Ministry official had estimated after the two days of voting ended on Thursday. However, it exceeded the 32.9 percent turnout in a referendum that backed the previous Islamist-tinged constitution under Mursi in 2012.

The new constitution, which won wide support from many Egyptians who favoured Mursi's removal, could lead to an outright ban on Islamist parties and strengthens the political grip of the already powerful military establishment.

BROTHERHOOD BOYCOTT

Mursi's Muslim Brotherhood, which has been designated a terrorist organisation and faces a fierce security crackdown, boycotted the vote and accuses the army of having staged a military coup against Mursi.

The army, Egypt's most powerful institution, said it was responding to the will of the people.

Hundreds of Mursi's supporters were killed when security forces stormed Cairo protest camps calling for his reinstatement in August and thousands more have been arrested since then, accused of violence or terrorism.

"The referendum took place against a backdrop of arrests and detention of dissenting voices," Democracy International, which deployed foreign observers across Egypt to monitor the polls, said in a statement on Friday.

"There was no real opportunity for those opposed to the government's road map or the proposed constitution to dissent."

The Muslim Brotherhood's political arm, the Freedom and Justice Party, called the referendum results invalid in a statement posted on its official Facebook page.

Cairo's main Western ally and military backer, the United States, has expressed concern for democracy since Mursi was ousted, but has also described Egypt, which has a peace treaty with Israel and controls the Suez Canal, as a vital partner.

"Egypt's turbulent experiment in participatory democracy in the last three years has reminded us all that it's not one vote that determines a democracy, it's all the steps that follow," U.S. Secretary of State John Kerry said in a statement.

"... it's what comes next that will shape Egypt's political, economic and social framework for generations."

YEARNING FOR CALM

The passing of the constitution reflects a widespread yearning for calm in the most populous Arab state, which has suffered from continual upheaval since a popular uprising toppled the ex-military autocrat Hosni Mubarak in 2011.

Mursi's opponents say the referendum will help to neutralise the Brotherhood, which had won every election since 2011 but became deeply unpopular after Mursi's year in office.

"The referendum is a blow that will expel them from the arena ... All those who go out in the streets must reassess. They cannot beat the will of 20 million," Deputy Prime Minister Hossam Eissa said on state television. - Reuters