Interior Minister Bayan Baker Solagh announced a series of measures ahead of today's vote, including a four-day national holiday through Sunday, a ban on civilians carrying weapons, and a ban on virtually all vehicle travel on Saturday.

Already on Friday the streets of Baghdad, normally choked with traffic, were surprisingly quiet.

Iraq's borders have been closed tight, with only vehicles transporting food, water and fuel exempted.

US ambassador Zalmay Khalilzad said that a successful referendum is a key component of a future US military withdrawal.

"Iraq's success is our exit strategy," said Khalilzad. US forces can leave the country "as soon as Iraq can stand on its own feet".

Khalilzad said the insurgency could be defeated only through a joint military, political and economic strategy. "I believe strongly that you cannot defeat the insurgency through military means alone," he said.

Five Iraqis, including four police officers, were killed Friday and six others injured in attacks across the country.

A car bomb was set off as a police convoy was driving near Baquba, 60km north of Baghdad, killing four police officers and injuring three others, police said. The officers were to be stationed outside a polling station.

In Baghdad, three civilians were wounded when a car bomb exploded as a US convoy passed by, a Ministry of Defence source said.

And two members of the Islamic Party, a predominantly Sunni group, that called for a yes vote on the constitution were attacked with explosives in Baghdad, though nobody was killed or injured.

Also in the capital, the US military said US soldiers were tipped off by locals about a powerful car bomb, which they found and disabled.

Soldiers found seven 122-millimeter artillery rounds, four 130-millimeter artillery rounds and a propane tank filled with homemade explosives inside the vehicle.

Separately US soldiers acting on a tip-off in the town of Taji, just north of Baghdad, discovered a mortar tube and 38 mortar rounds. They shot dead a man who threatened them with an AK-47 when they moved in to the area.

Both events happened Wednesday but were reported by the US military Friday.

In Washington, US State Department deputy spokesman Adam Ereli said Thursday that preparations for the vote were going "very well" and suggested the level of violence was lower than before the last elections in January.

Ereli was unable to give figures to back up his assertion about the bloodshed however, and under persistent questioning appeared to give ground.

"The number of attacks, in our view, is not the measure of the success or the strength of democracy in Iraq," he said.

"I don't think anyone is under any illusion that the attacks are about to end or that we're turning an important corner."

However in Baghdad, Major General Rick Lynch, spokesman for the US-led multinational forces in Iraq, emphasized that the number of insurgent attacks had dropped compared to the period leading up to the January polls, which elected the current legislature.

US and Iraqi forces have killed or captured 91 people said to be leaders of al-Qaeda in Iraq, he said. "We believe that decimating their leadership has had a significant impact on the number of attacks," he said.

Lynch added that US-led multinational forces and Iraqi forces were nevertheless bracing for an increase in attacks linked to the referendum.

al-Qaeda-claimed attacks killed more than 90 people in three days earlier this week, including 30 on Wednesday in a suicide attack on a recruitment centre and 35 on Tuesday in an open market bomb attack, both in the northwestern town of Tal Afar.

Reported attacks dropped considerably Thursday and early Friday.

In Washington, a quarterly Pentagon report showed that as of September 16, the number of attacks by Iraqi insurgents was averaging nearly 600 per week, compared with just over 500 around the country's January elections.

Saturday's referendum is being held three years after Saddam -- who is awaiting trial on charges of crimes against humanity -- was re-elected for the last time in a poll that showed 100 percent of the vote in his favour.

He and seven former officials go on trial next Wednesday for the 1982 killing of 143 Shiite villagers.

Some 15.5 million Iraqis are registered to vote from the population of 26 million.

Lawmakers endorsed last-minute changes Wednesday to the draft charter in a bid to ease bitter ethnic divisions ahead of the vote, but several Sunni groups still held to their staunch opposition.

Meanwhile, an Iraqi Sunni cabinet minister said he expected the nation to reject the constitution in today's referendum, saying the draft did not meet the needs of the entire country.

"It does not meet the interests of all Iraqis," Industry Minister Osama Abdul Aziz al-Najafi said during a visit to Malaysia. "We need something to be amended. The majority of the Iraqis don't accept it."

Sunnis, who comprise about 20 percent of Iraq's 26 million population, fear that federal provisions in the draft could lead to the break-up of Iraq and leave control of its vast oil wealth in the hands of Shiites and Kurds.

Iraqi lawmakers had endorsed last-minute changes to the draft charter in a bid to ease bitter ethnic divisions ahead of the constitutional referendum, but several Sunni groups still held to their staunch opposition.

"Iraq now is facing a critical situation that needs support from friendly countries and the Islamic community so as to maintain ... its sovereignty and the unity of its land," he said.