Radical Iraqi Shia cleric Moqtada Sadr has appealed for calm and an end to clashes between rival Shia factions.
Moqtada Sadr commands deep loyalty among many poor Shias
On Wednesday, seven people died in an attack on Mr Sadr's office in Najaf, leading to unrest in other cities.
BBC Baghdad correspondent Mike Woodridge says it is not clear what lies behind the violence.
Mr Sadr has sided with Sunni leaders in opposing the federalist tone of the draft constitution, warning that this could lead to the break-up of Iraq.
The violence has added to the tensions as political leaders in Iraq are under pressure to agree the new constitution before a deadline on Thursday night.
Mr Sadr's supporters blamed the clashes on a rival pro-government Badr militia.
A representative of the Badr militia, loyal to the largest Shia party, the Supreme Council for the Islamic Revolution in Iraq (Sciri) led by Abdul Aziz Hakim, condemned the Najaf attack of Mr Sadr's offices, which was burned down in the clashes.
In a televised address late on Wednesday, Iraqi Prime Minister Ibrahim Jaafari, also a Shia, condemned the attack.
Mr Sadr's followers staged two revolts against US troops in 2004.
His office in Najaf had been closed down since his group agreed to a cease-fire last year, but reports say Mr Sadr was attempting to reopen it.
In response to the attack on the office, Mr Sadr's followers, known as the Mehdi Army, mobilised supporters elsewhere in Iraq, including Baghdad's Sadr City slum, a radical stronghold in the capital.
A spokesman for the Mehdi Army said the militia had moved to a state of "high alert" after the violence.
A wave of reprisals were reported against Badr targets in Baghdad and in other Shia towns.