Iraq's most senior Shia cleric has called on the country's majority Shia community to vote "yes" in Saturday's constitution referendum.
Sistani holds sway over a large section of Iraqi society
The statement was conveyed by aides of Grand Ayatollah Ali Sistani based at the shrines of Najaf and Karbala.
It is his most direct show of support for the charter and it is expected to carry great weight with Iraqi Shias.
Iraqi and US forces have stepped up security across Iraq to stop insurgent attacks aimed at wrecking the vote.
A four-day public holiday has been called and the streets of major cities are unusually quiet with schools and offices closed.
A night curfew is also being imposed and on Friday Iraq's external and internal borders will be sealed, allowing no travel between provinces.
Concrete and barbed wire barriers are being erected around polling stations in order to stop suicide car bombers, who have caused devastation in recent days.
In Mosul, police have gone further, banning all civilian vehicles from the roads.
Meanwhile, thousands of Iraqi detainees in US and Iraqi-run jails have begun voting on the draft constitution.
It has not been disclosed known whether the former Iraqi leader, Saddam Hussein - whose trial for crimes against humanity begins on 19 October - will be allowed to vote.
Correspondents say the backing of Ayatollah Sistani was one of the driving forces in January elections that put Shia religious parties and secular Kurds in control of the parliament.
"The Iraqi faithful must go to the polling stations to vote in favour," said Sayed Ahmed Safi in Karbala.
He said any possible objections had been removed by last minute changes to the text endorsed by parliamentary on Wednesday.
Political leaders have announced their agreement to consider further revisions after elections in December and certain key additions to the 139-article draft constitution, including:
- Article 1 which now reads: "This constitution is a guarantee for the unity of Iraq."
- Article 4 which now reads: "The federal and official organisations in the Kurdistan region use the Arabic language and the Kurdish language as the official languages."
- Article 131 which now reads: "Just being a member of the Baath Party is not grounds for prosecution and any [former] member is treated equally before the law and has all protections of the law."
Sunni leaders fear the current proposals may lead the country to split into a Kurdish north and Shia south, depriving Sunnis Arabs of access to the country's oil resources.
About 12,000 prisoners may be allowed to vote
One of the main Sunni parties, the Iraqi Islamic party, said the changes meant they could call on their supporters to back the constitution.
But they were not enough for many Sunni leaders, for example the influential Association of Muslim Scholars.
A spokesman Abdulsalam al Kubaisi told a news conference he was disappointed in the Iraqi Islamic Party and accused the party of violating an agreement with other Sunni groups.
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 the constitution.
However, correspondents say it will be difficult to attain such a vote and it is more likely the constitution will be approved.