Sectarian violence continues to torment Iraq as politicians wearily prepare to debate the country's draft constitution again ahead of tonight's midnight deadline.

Forces of rival Shia factions clashed violently overnight in several southern cities, as militiamen loyal to Hojetoleslam Moqtada al-Sadr took revenge after their local office in the holy city of Najaf was burned down. Seven people were killed and dozens wounded in the fighting.

Meanwhile six elderly men were shot dead at breakfast in a cafe north of Baghdad, in an attack blamed on extremist Sunnis who are trying to impose strict behaviour rules through the barrel of a gun.

Hojetoleslam al-Sadr, who is unusual among Shia leaders in not supporting the draft constitution, claiming - like Sunni Arab opponents - that it is a recipe for the break up of Iraq, ordered his followers to go home and calm down to allow the political process to continue.

"I call upon all believers to spare the blood of the Muslims and to return to their homes," the cleric told reporters in a press conference at his Najaf base.

"I will not forget the attack on the office... but Iraq is passing through a critical and difficult period that requires unity."

He blamed the burning down of the office on supporters of the Supreme Council for the Islamic Revolution in Iraq, the Iranian-backed Shia group known as Sciri which has a powerful bloc in parliament. Hojetoleslam al-Sadr is an Iraqi nationalist, scornful of those Shias who fled abroad under Saddam and took refuge in neighbouring Iran.

He called for Abdul-Aziz al-Hakim, the leader of Sciri, to condemn "what his followers have done". Sciri denied burning down the office.

Far from dying down, by this morning the unrest had spread to Diwanayah, where Hojetoleslam al-Sadr's Mehdi Army militiamen occupied part of the city centre and fired on police and rival groups, impelling residents to flee. They also burnt down an office of Sciri's Badr Brigade militia in the area of Baghdad known as Sadr City.

But the most savage incident today was the discovery of the bodies of 36 men at 9.30am this morning in a dry river bed near the Iranian border, police said. All had been shot in the head and some were handcuffed.

Col. Ali al-Kuraishi estimated that the men were killed within the last three days since one had a paper in his pocket dated last Monday. None had any identification cards. The bodies were transported to a morgue in Kut.

The area southeast of Baghdad has witnessed killings in the past between Shias and Sunni Muslims, but all but one of the victims was wearing traditional Kurdish trousers.

The violence has cast a pall over the negotiations about Iraq's future. The politicians need to meet tonight's deadline, the latest for Iraq's National Assembly to approve the controversial draft constitution, if the process of transferring power to a democratically elected Iraqi government is to remain on track.

Under a tight schedule, a referendum on the draft constitution is due to take place within two months, on October 15, followed by fresh national elections in December.

The Sunni minority - already frustrated by their lack of representation in Parliament - are afraid that demands for self-rule by the prosperous Shia south and the oil-rich Kurdish north will leave them possessed of nothing but the poverty-stricken central plains of Iraq, where the insurgency is raging out of control.

The interim government moved quickly to try to stop the Shia-on-Shia violence escalating. President Jalal Talabani, a Kurd, telephoned Hojetoleslam al-Sadr this morning to ask for restraint. The country's prime minister, Ibrahim al-Jaafari, has apparently offered to ensure the office in Najaf is rebuilt and better guarded.

Meanwhile Ahmad Chalabi, the deputy prime minister and great survivor of Iraqi politics, who has been cultivating Hojetoleslam al-Sadr while keeping up his close links with Iran, condemned the attack that started the violence and promised "an immediate investigation".

In his press conference, Hojetoleslam al-Sadr criticised the draft constitution, which the Sciri parliamentary bloc is proposing to ram through against opposition. He said that it was not tough enough in punishing Saddam's former ruling Baath party, and would lead to a federal Iraq.

"We reject federalism and if America has schemes, it should not try to implement those schemes at once," he said.

Sunni Arabs said today that the draft constitution should be ditched and new elections should be held in an attempt to get a document which could win consensus.

Hussein al-Falluji, one of the 15 Sunnis on the 71 member drafting panel, threatened to go to court if the document is passed by parliament tonight. He said that it was invalid because parliament failed to meet an earlier deadline.

"The draft is illegal and it is a violation of the interim constitution," said Mr al-Falluji. "We will take this to a federal court if the constitution passes through parliament, which by law should be dissolved since the deadline was not met."

When the draft was finally presented to parliament on Monday, a week late, Sunnis fiercely rejected it and predicted it would lead to more violence. As a result, the Shia and Kurdish leaders agreed to delay the vote on it until today, in the hopes of achieving a greater degree of consensus.

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