The military-installed government has implored voters to turn out en masse to ratify the constitution in the two-day referendum, hoping to bolster its disputed authority after Morsi's ouster.

An Islamist coalition led by Morsi's Muslim Brotherhood has called for a boycott and “civilised peaceful protests” during the polling, and the interior ministry has pledged to confront attempts to disrupt voting.

The small, improvised bomb exploded less than two hours before polls opened, causing little damage and no injuries, a police general said.

But it again highlighted the government's precarious grip on the most populous Arab country, still reeling from Morsi's ouster and a bloody crackdown on his Islamist supporters.

The police and army have deployed hundreds of thousands of police and soldiers to guard polling stations amid fears that a spate in militant attacks and protests would keep voters at home.

At one polling station for women at a school, dozens lined up to cast their ballots, some waving Egyptian flags and chanting pro-military slogans.

While it is uncertain how many Egyptians will vote amid concern over violence, the constitution appears certain to pass.

The charter has done away with much of the Islamist-inspired wording of Morsi's constitution, which was suspended on his overthrow, and its supporters say it expands women's rights and freedom of speech.

But it has bolstered the military's powers, granting the army the right to appoint the defence minister for the next eight years and to try civilians for attacks on the armed forces.

Interim president Adly Mansour urged Egyptians to turn out at the polls.

“I call on you to live up to the responsibility you owe to your nation and to ensure a better future for this country to go to your polling station and vote,” he said in a speech on Sunday.

The runup to the vote has been marred by a deadly crackdown on Morsi's supporters, and arrests of activists who campaigned for a “no” vote.

At least seven activists have been detained in the past week as they distributed posters or leaflets critical of the new constitution, prominent rights lawyer Ragia Omran told AFP, adding that most were released after a few days.

The capital has been festooned with banners urging Egyptians to vote “yes”, often featuring military motifs such as a general's hat, an allusion to army chief Abdel Fattah al-Sisi.

Many Islamists revile Sisi as the man who overthrew the country's first freely elected and civilian president, but the general is adored by the millions who took to the streets in July to demand Morsi's resignation.

The army chief is widely expected to run for president, and has said he would stand for election if he felt there was “popular demand,” state media reported this week.

Sisi will closely monitor the turnout and result of the referendum as an “indicator” of support for a presidential bid, said an official close to the general.

The authorities are worried that a low turnout would empower their Islamist opponents in Morsi's Brotherhood movement and cast further doubt on their legitimacy, analysts say.

At least 1,000 people, mostly Islamists, have been killed in street clashes, and thousands have been imprisoned since the ouster of Morsi, whose supporters continue protests almost every day.

Source: AFP