Polls are closing in Iraq where people have voted in their millions to decide a constitution seen as vital to maintain the unity of the country.
The vote is seen as a key step in bringing stability to Iraq
Vehicles were banned from roads to prevent car bombings during the vote and troops were on alert.
Despite this, three Iraqi soldiers were killed in a roadside bombing north of Baghdad and several civilians were wounded in other violent incidents.
Kurds and Shias back the charter, which is opposed by most Sunni Arabs.
Up to 15.5 million voters had been expected to register their view on the charter before polls closed at 1700 (1400 GMT).
Turnout among Sunni Arabs is reported to be higher than during the general election earlier this year.
Leading Sunni figure Saleh al-Mutlaq told the BBC that Sunnis wanted to reject the draft constitution which they felt would break up the country.
President Jalal Talabani and Prime Minister Ibrahim al-Jaafari were among the first to vote, and pictures of them inside the fortified Green Zone was shown live on Iraqi television.
"I voted Yes and I urge all Iraqis, no matter their different ethnicities and religions... to vote Yes to the constitution," Mr Talabani told reporters.
Leaders of the main Shia religious and Kurdish parties are backing the constitution, which was drafted by a committee in which their representatives predominated.
CONSTITUTION'S KEY POINTS
Iraq to be federal, parliamentary democracy
Official languages to be Arabic and Kurdish
Official religion to be Islam but religious freedoms guaranteed
Equal rights for all
Elections every four years
Sunnis - who largely boycotted the January vote that paved the way for the constitution - fear it may lead the country to split, with a Kurdish north and Shia south, depriving Sunni Arabs of access to the country's oil resources.
Insurgents targeted an Iraqi military convoy near the Iranian border east of Baquba, killing three soldiers, and at least six civilians were injured in attacks on polling stations in Baghdad.
But conditions in many areas were described as calm - in marked contrast to the violence that erupted during January's poll when insurgents carried out dozens of attacks.
Security has been tight across Iraq. A four-day public holiday is under way, shutting government offices and schools. Baghdad airport has also been closed.
Borders have been closed to non-essential traffic.
Private cars were banned from roads in many areas, and voters had to undergo security checks before entering heavily-guarded schools-turned-polling stations.
Voting between 0700 and 1700 (0400-1400GMT)
15.5m Iraqis registered to vote
Security measures: Four-day public holiday, traffic ban and night curfews
Overnight, insurgents attacked power lines leading to the capital Baghdad causing black-outs, but electricity was restored before dawn.
The election commission said it had set up more than 6,000 polling stations, including in the regions worst-affected by violence like the vast Anbar province, which is seen as the heartland of the insurgency.
In Washington, US Vice-President Dick Cheney said the US expected the constitution to be approved. Anti-US insurgents had vowed to disrupt the vote.
Unidentified gunmen attacked three offices of the largest Sunni Arab political party, the Iraqi Islamic Party, after it dropped its opposition to the draft text.
If voters in three of Iraq's 18 provinces muster a two-thirds majority against the constitution, it will fail.
The Sunnis are dominant in four provinces and so therefore effectively hold a power of veto if they turn out in large numbers to vote against it.
If the text passes, it will provide the basis for parliamentary elections.
The results of the referendum will not be declared for several days.