Egyptians are voting on a new constitution amid high security in a referendum likely to prompt a presidential bid by the army chief who overthrew Islamist president Mohamed Morsi.
The military-installed government implored voters to turn out en masse on Tuesday 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 Morsi'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 Morsi 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.
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Shortly before polling stations opened, a small, improvised bomb exploded outside a Cairo court, damaging the facade but causing no injuries, 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 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.
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.
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.
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 said, adding that most were released after a few days.
