The fresh outbreak on Thursday of inter-Shia violence, on top of a bloody show of force by fighters on Baghdad's streets on Wednesday, has cast clouds on Iraq's constitutional process.
One man was killed and 13 wounded in overnight clashes between al-Sadr supporters and members of the rival Supreme Council for the Islamic Revolution in Iraq (SCIRI) in the southern town of Nasiriya, 375km southeast of Baghdad.
By daybreak on Thursday, residents of several Shia southern cities reported burned out political offices; in Amara, near Basra, a policeman was killed in dawn clashes with al-Mahdi Army fighters who had occupied the local premises of the rival Badr organisation overnight, a local government official said.
Al-Sadr, a strident nationalist whose followers deride rival Shia leaders for their time in exile in Iran, has joined Sunni Arab leaders in denouncing the draft constitution as a recipe for the break up of the state.
Sadr is opposed to the draft
Washington has pressed for the charter's adoption as part of a strategy to eventually pull out its troops. It announced plans to send 1500 elite soldiers as part of possibly 20,000 extra forces to police the referendum and a December election.
Parliamentary officials said the National Assembly would meet, but it was not clear when.
Accounts of an attack on a al-Sadr office in Najaf were patchy and confusing. But it was followed after dark by more clashes there, in at least three Shia districts of Baghdad and in towns across the Shia south.
Call for calm
"I call upon the people of Iraq, ... the believers ..., to preserve the blood of Muslims and go back home. I call upon them to maintain calm," al-Sadr said at a press conference on Thursday.
He wanted rival Shia leader Abdul Aziz al-Hakim to condemn his followers for their part in clashes.
But Sadr also had conciliatory words for Prime Minister Ibrahim Jaafari, who has called for calm and Shia solidarity.
A spokesman of al-Sadr warned of a "general call to arms" if there were no apology, and criticised Najaf's governor, a member of the Supreme Council for the Islamic Revolution in Iraq (SCIRI).
Al-Sadr supporters said his office in Najaf was targeted. It had reopened this week after a year's closure since a US offensive in the city had put down the second of two uprisings the young cleric led against the US-led presence.
The Badr movement has denied
any role in the violence
One of his followers, who sits in the government as health minister, said eight people were killed inside the office.
Another said the building had been burned down. Dozens of people were wounded in subsequent clashes, hospital officials said.
Language of the gun
The head of the Badr movement, SCIRI'S armed wing from its days in Iranian exile, denied involvement in the attack and called it a "criminal aggression" aimed at dividing Iraq's Shia majority.
There were reports of violence at Shia political offices in Basra, Nasiriya, Hilla, Samawa, Diwaniya and Amara.
Interior Minister Bayan Jabor, from SCIRI, went on state television to say he had dispatched police commandos to Najaf, 160 km south of Baghdad, and imposed a curfew.
Opponents complain that the police have recruited many Badr members.
"The gun and the language of the gun must end"
Iraqi Prime Minister
Prime Minister Ibrahim Jaafari, from another Shia Islamist party Dawa, made an impromptu live television address after midnight to urge for calm and to praise the Sadr movement.
"I condemn these events that target offices of the religious authorities ... I promise an investigation to find out who was behind this incitement," Jaafari said.
"The gun and the language of the gun must end," he said, calling on Iraq's newly empowered Shia to remember the repression they suffered under Saddam Hussein.
Al-Sadr supporters have demonstrated against the draft constitution in recent days, and some joined Sunni demonstrators in a march on Wednesday, part of efforts to mobilise a blocking vote against the charter in the referendum.
Sunnis, largely missing from parliament after failing to vote in national elections in January, say a proposed federal system could hive off Iraq's oil wealth to autonomous Kurdish and Shia regions in the north and south respectively.
The constitution will fail if two thirds of voters in three of Iraq's 18 provinces vote "No". That could happen in three Sunni provinces. The government has been struggling to staunch discontent even in its southern heartlands, where large protests over unemployment and poor local services have taken place.
Sadr, who is barely in his 30s, derives strength from poor Shia and the reputation of his father, killed under Saddam in 1999.