The text read to parliament on Sunday failed to overcome objections by Sunnis, who lost their political dominance with the fall of president Saddam Hussein, despite intense US efforts to broker a compromise between Iraq's divided ethnic and religious groups.
The US, which sees approval of a constitution as key to defusing an insurgency, welcomed the draft, hailing it as a victory for democracy.
Rejection in the three of Iraq's 18 provinces dominated by Sunni Arabs would be enough to torpedo the constitution under current referendum rules.
President Jalal Talabani urged Iraqis to vote 'yes' in the referendum - due by 15 October.
"We hope that this constitution will be accepted by all Iraqis and that it will be for everybody. We are optimistic," Talabani, a former Kurdish forces leader who fought Saddam Hussein, said.
A Sunni Arab delegate on the drafting committee said all his colleagues on the panel objected to the draft presented to parliament.
Iraq's constitution drafting panel
has signed the draft charter
"We have not agreed on this constitution. We have objections which are the same as we had from day one," said Hussein al-Falluji, the Sunni Arab delegate.
"If there is no forging of the results, I believe the people will say 'no' to the 'American' constitution," he said.
US President George Bush on Sunday touted Iraq's new constitution but acknowledged Sunni opposition and that the referendum could spark a new wave of violence.
"Of course, there's disagreements," said Bush, who had made a personal appeal to Iraqi Shia to cut a deal with Sunni Arabs. "That's their right."
Bush urged Iraqis to debate the constitution on its merits and sang the document's praises, saying it "contains far-reaching protections for fundamental human freedoms including religion, assembly, conscience and expression".
But he warned: "We can expect ... atrocities to increase in the coming months because the enemy knows that its greatest defeat lies in the expression of free people in freely enacted laws and at the ballot box."
Although Iraq's parliament adjourned without a vote, delegates in the Shia- and Kurdish-dominated assembly said the draft's reading in parliament was enough to signify its acceptance.
"We tried hard to include everybody's demands but this could not be done. Some people are still opposed to some points. Even I may have some reservations," said parliament Speaker Hajim al-Hassani.
"But now we should think of this country and its unity. Whoever wants to change something, then the referendum is the final chance. Iraqis should prepare themselves for elections."
Iraq will hold elections in December after the referendum.
Iraqi parliament Speaker Hajim
al-Hassani appealed for unity
The text read in parliament suggested limited concessions to the Sunnis.
Retreating from earlier drafts referring to Saddam's political party, it omitted the phrase "Baath party" and instead banned "the Saddamist Baath and its symbols".
Sunnis had pressed for the removal of any clauses in the draft that bar party members from public life, arguing that not all of them have blood on their hands.
The text sticks to wording that says Iraq is "part of the Islamic world and its Arab people are part of the Arab nation".
Sunnis, and some Shia, who are also Arabs, wanted it to say that Iraq as a whole is part of the Arab world. The Kurds of the north are Muslims but not Arabs.
The preamble made clear that Iraq was a federal republic.
Sunnis' main objection has been to federalism, which they fear could lead to the break-up of the country and leave them with a rump state minus the rich oil zones in the north and south.
Forcing the pace as he has done for the past month to keep Iraqi leaders to a US-sponsored timetable, Washington's envoy Zalmay Khalilzad was an ubiquitous presence in the meeting rooms of Baghdad's fortified Green Zone on Saturday.
US envoy Zalmay Khalilzad called
the document 'progressive'
Khalilzad hailed the document, saying: "The draft constitution is one of the most progressive governing documents in the Muslim world in terms of its protections of the rights of religious freedom and consciousness."
But he said he was disappointed by the Sunni reaction.
"I understand their circumstances. They are in a difficult position. There are threats of intimidation. You've seen some of them saying that they like the document, but they're afraid if they openly support it, their lives could be at risk."
Aljazeera quoting the US army reported that a soldier was wounded when a bomber blew up his car near a US convoy in the northern Iraqi city of Mosul.
Iraqi police sources said three people were killed and four wounded in the blast.
In the city of Kirkuk, the US army said an armed man was killed and another wounded when US and Iraqi forces stormed a house.
A firefighter puts out a blaze at
a blast site in Mosul on Sunday
Near the western town of Falluja, Iraqi police found three bodies of police officers in the al-Thirthar area northwest of the town.
In Samarra, two Iraqis were killed by armed men in the centre of the city.
In al-Ishaqi town, north of Baghdad, a truck driver who was among a convoy transporting supplies for the US army was killed.
In Baghdad, a civilian was killed and another wounded by armed men in al-Dora district.
An Iraqi journalist working for a local TV channel and an Egyptian national working for a sports club were shot dead in the northeastern Iraqi city of Baquba.