A first day of voting has been taking place in Egypt on a new constitution that could pave the way for fresh elections.
But clashes involving supporters of ousted Islamist President Mohammed Morsi led to several deaths.
The new charter is to replace the constitution passed under Mr Morsi before he was forced out by the army.
The military wants a strong Yes vote in the two-day referendum to endorse his removal.
Mr Morsi's Muslim Brotherhood, now designated a terrorist group, is boycotting the vote.
His supporters clashed with security forces in several parts of Egypt and officials said nine people had died:
Four people were killed and more wounded in clashes in the Upper Egypt city of Sohag, though details of the incident are disputed
One person died in Nahia, in the Giza district of Cairo
Another was killed during an anti-referendum protest in Bani Suef, south of Cairo, the governor there told the BBC
Three people - Morsi supporters according to security sources - are reported to have been shot dead in the Cairo suburb of Kerdasa
Shortly before voting began, there was an explosion near a court building in Cairo's Imbaba district, although no casualties were reported.
A huge security operation is being mounted for the two days of voting. Some 160,000 soldiers and more than 200,000 policemen are being deployed nationwide.
Army chief Gen Abdel Fattah al-Sisi, wearing dark sunglasses and khaki fatigues, visited one polling station in north Cairo, telling guards there: "Work hard. We need the referendum to be completely secured."
The BBC's Orla Guerin in Cairo says this has been a distorted campaign, with endorsements for the new constitution flooding state-run and private TV and radio.
However, spotting any posters from the No campaign is a lot harder and people have been arrested for putting them up, our correspondent says.
Democratic or not, she says, the referendum is seen by many as more than a ballot on a new constitution - it is widely viewed as a verdict on the removal of Mr Morsi.
State-run media were on Tuesday describing the vote as a "democratic ceremony" - a term widely used during the Hosni Mubarak era but not heard since he was ousted in the revolution of January 2011.
One voter in Cairo, Salah Mustafa, told the BBC: "Compared with the document that we had last year, which was a really horrible constitution, there's a lot of rights."
But Mohammed Soudan, a spokesman for the Brotherhood's political wing, said most people were boycotting the vote, adding: "This is a message that we are not recognising this kind of new power."
Interim Prime Minister Hazem Beblawi has called the referendum the "most critical moment" for Egypt.
Interim President Adly Mansour said after voting: "The people must prove to dark terrorism that they fear nothing."
The new constitution was drafted by a 50-member committee that included only two representatives of Islamist parties.
The authorities maintain that it is a crucial step towards stability.
Under the new constitution:
The president may serve two four-year terms and can be impeached by parliament
Islam remains the state religion - but freedom of belief is absolute, giving some protection to minorities
The state guarantees "equality between men and women"
Parties may not be formed based on "religion, race, gender or geography"
Military to appoint defence minister for next eight years
Critics say the new constitution favours the army at the expense of the people, and fails to deliver on the 2011 revolution.
A Yes vote could lead to fresh elections and it now seems certain that Gen Sisi, who backed Mr Morsi's removal following mass protests, will run for president.
The constitution is expected to attract a resounding Yes vote, but the turnout is key, analysts say.
The last charter, passed just over a year ago, was approved by 63.8%, but only 32.9% of the population voted.
Mohammed Morsi was Egypt's first democratically elected president but was deposed by the military last July.
He is being held in jail in Alexandria, facing several criminal charges relating to his time in office - which he says are politically motivated.
More than 1,000 people have died in violence since Mr Morsi's overthrow.