US and Iraqi officials on Sunday predicted Iraq's draft constitution was heading for approval, lending further legitimacy to Iraq's political process. But as election officials counted the ballots from Saturday's referendum, opponents of the document were also confident, saying the unexpectedly high turnout in Sunni areas of the country would bring about the document's defeat.

Though the final results may not be announced before the end of the week, Condoleezza Rice, the US secretary of state, said in London: “Most people assume on the ground that it [the constitution] probably has passed.”

Her comment drew immediate criticism from Saleh al-Mutlek, a Sunni politician who helped lead the campaign against the constitution.

He claimed early reports suggested that two-thirds of voters in at least three largely Sunni Arab provinces had come out against the draft, which would be enough to defeat it.

In Baghdad, the referendum was free of many of the jitters, but also the excitement, of January's elections, the first nationwide vote in post-invasion Iraq.

Some election officials said 61 per cent of voters had turned out, up from 58 per cent in January. But Iraq's Independent Electoral Commission stressed on Sunday that no official figures had been released.

If the draft passes, elections in December will usher in Iraq's first permanent postwar government. If it fails, the country will go through another year of transitional rule as parliament attempts to draft the constitution anew.

Many Sunni Arabs, who make up 20 per cent of Iraq's population, said they opposed the draft which was largely produced by the main Shia and Kurdish political parties because it would divide the country or bring about Shia or Iranian domination.

Observers in Sunni provinces reported large crowds of No voters flocking to the polls, with turnout rates of up to 80 per cent reported in some districts in the north compared with single digits in many parts of the country in January.

In contrast, many Shia and Kurdish areas saw a less enthusiastic turnout than in January, with some voters possibly discouraged by the failure of January's elections to bring stability or simply confident that the passage of the constitution was a foregone conclusion.

Some Shia were also sceptical of a document produced with heavy US input.

The office of the radical Shia leader Moqtada al-Sadr issued a last-minute call to his followers to cast their ballots against the document.

Western officials have expressed concern that a close-run vote will demonstrate that there is no national consensus, which could possibly cause further polarisation between the country's ethnic and sectarian groups.

However, the high Sunni turnout could also be encouraging news for US and Iraqi officials who have expressed hopes that if the Sunni Arab community turns to peaceful politics, it will undercut the insurgency.

The referendum campaign will be followed almost immediately by preparations for elections in December. Sunni Arab politicians say they are involved in at least two different sets of negotiations to form electoral coalitions in order to contest the vote.