CAIRO (AFP) - Egyptians were voting on a new Constitution on Tuesday amid high security in a referendum likely to prompt a presidential bid by the army chief who overthrew Islamist president Mohamed Mursi.

The military-installed government implored voters to turn out en masse to ratify the constitution, with the country's lingering polarisation underscored by the explosion of a small bomb in Cairo that caused no injuries.

An Islamist coalition led by Mursi's Muslim Brotherhood has called for a boycott and "civilised peaceful protests" during the two days of polling, and the Interior Ministry has pledged to confront attempts to disrupt voting.

Defence minister Abdel Fattah al-Sisi, the general who overthrew Mursi in July, visited a polling station at a north Cairo school after voting began to survey the security preparations.

"Work hard. We need the referendum to be completely secured," he told soldiers guarding the school.

Shortly before polling stations opened, a small, improvised bomb exploded outside a Cairo court, damaging the facade but causing no injuries, the police said.

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

The government hopes a large turnout in favour of the charter will bolster its disputed authority, while Gen Sisi will monitor it for an "indicator" of his popularity, an official close to the general said.

Interim Prime Minister Hazem al-Beblawi entreated voters to cast their ballots.

"Our country needs every vote from Egyptians," he said after casting his ballot, state media reported.

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 Mursi'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.

The runup to the vote has been marred by a deadly crackdown on Mursi'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 Gen 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.

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

Backers of the constitution are hoping for at least a 70 percent vote in favour of the Constitution as a satisfying majority.

Mursi's heavily Islamist influenced constitution passed with 64 percent of the vote, but on a turnout of barely 33 per cent of the country's 53 million voters.

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