Parts of the Iraqi draft constitution are a "recipe for chaos", Arab League Secretary General Amr Moussa has said.
Sunnis, once dominant, refused to endorse the text
He told the BBC the Arab League shared Sunni Muslim concerns over federalism and the fact the charter does not identify Iraq as an Arab country.
The US and UK have played down Sunni leaders' rejection of the text, which will go to a referendum by 15 October.
About 2,000 Sunnis in Saddam Hussein's home town of Tikrit took to the streets to voice their opposition to the plan.
Many waved pictures of the former Iraqi leader, and a statement was read out denouncing the constitution as a "Jewish" document that would divide Iraq along sectarian lines.
In Baghdad, thousands of Shias gathered to show their support for the draft constitution, which offers a measure of autonomy for millions of Shias in the oil-rich south.
Plans are being made to distribute copies of the text around the country.
Shia and Kurdish negotiators have endorsed the document, but negotiators from the Sunni minority refused to put their names to it.
They fear the proposals would lead to the break-up of the country into a Kurdish north and Shia south, depriving the Sunnis of access to the country's oil resources.
Mr Moussa, of the Arab League, told the BBC's World Today programme: "I share the concerns of many Iraqis about the lack of consensus on the constitution."
He said he was concerned that the draft text denies Iraq's "Arab identity".
Federalism, and forming of semi-autonomous regions
Terminology used to eradicate influence of former Baath regime
Structure of authority between presidency, parliament and government
"I do not believe in this division between Shia and Sunni and Muslims and Christians and Arabs and Kurds," he said. "I don't buy this and I find in this a true recipe for chaos and perhaps a catastrophe in Iraq and around it."
The text of the draft constitution says Iraq is "part of the Islamic world and its Arab people are part of the Arab nation" - an apparent concession to non-Arab minorities like the Kurds.
Sunni negotiators wanted the text to say that Iraq as a whole is part of the Arab world.
US President George W Bush welcomed the completed draft constitution and said the referendum was a chance for Iraqis to "set the foundation for a permanent Iraqi government".
"Of course there's disagreement. We're watching a political process unfold, a process that's encouraged debate and compromise," he said.
But it is possible the document may never come into force. To be ratified, the constitution has to be approved by a majority of voters across the country - as well as not being rejected by two-thirds of voters in at least three or more of Iraq's 18 provinces.
The Sunnis are dominant in four provinces and thus in effect have a power of veto, correspondents say.
Iraq's President Jalal Talabani also called on all Iraqis to support the draft document.
15 August deadline extended twice
National referendum on constitution by mid-October
Full government elections by mid-December
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"We hope that this constitution will be accepted by all Iraqis and that it will be for everybody," he said.
"For sure there is no book that is perfect and cannot be amended except the holy Koran."
Among the Sunni concerns are provisions in the constitution which exclude former Baath party officials from public office and which pave the way towards federalism.
They are concerned that allowing for federalism may lead to the creation of an autonomous Shia area in southern Iraq - like the Kurdish north but under Iran's influence.
However, the Sunni representatives said they would play an active role in the elections scheduled for December and called on all Sunnis in Iraq to register to vote.