Fighting broke out in Baghdad and the holy city of Najaf tonight between rival Shi'ite militias, raising fears of a renewed uprising by radical cleric Moqtada al-Sadr's Mehdi army against the US-backed government.
At least eight people were killed and dozens wounded, health officials said, in street battles in Najaf involving pro-government Badr Organisation fighters and supporters of Sadr, who has joined Sunni Arabs in denouncing a constitution the Shi'ite-led government is preparing to force through parliament.
The head of the Badr Organisation denied it was involved.
The interior minister dispatched police commandos to Najaf and announced a curfew in the city on state television.
A spokesman for Sadr warned of a "general call to arms" unless rival groups apologised for what he called attacks on Sadr's office in Najaf. His Mehdi Army was banned after US troops crushed two uprisings last year, but it has not disarmed.
Earlier dozens of insurgents armed with rocket-propelled grenades and assault rifles attacked police checkpoints in western Baghdad, in some of the heaviest street fighting the capital has seen in months.
Several loud explosions shook the Hay al-Jamia district of the city and at least six police vehicles were set ablaze as a group of around 40 insurgents, some with their faces masked, launched a brazen daylight assault, police and witnesses said.
"It was raining bullets," said a police official, who said a dozen police vehicles had been sent in to try to evacuate forces, but had failed against the onslaught of gunfire.
A police source said three police officers and three civilians were killed and 31 people were wounded, most of them civilians. Police shot dead two attackers, detained two others, and seized four cars filled with weapons, the source said.
Iraqi police official
A police commander said Iraqi security forces had called for US military reinforcements. The attack came a day before parliament was due to vote on a new constitution backed by the Shi'ite-led government but opposed by Sunnis.
Sunni Arab insurgents have threatened to kill anyone taking part in politics, including the drafting of the constitution. Hay al-Jamia is close to the Amiriya district, an area that has been a stronghold of the insurgency over the past two years.
Iraqi security forces conducted raids in Amiriya and other predominantly Sunni neighbourhoods on last night, arresting several suspected insurgent ringleaders.
A senior US military official said on Wednesday his forces were bracing for an insurgent onslaught as the Shi'ite and Kurdish-dominated parliament prepared to adopt the constitution.
"We believe that the enemy is still ... intending to conduct some larger-scale operation in Baghdad associated with the release (of the constitution)," he said.
Iraqi President Jalal Talabani held another day of talks with leaders from the Sunni, Shi'ite and Kurdish communities on Wednesday to try to forge a consensus on the charter, but he looked unlikely to succeed with a day to go before a vote. Sunni leaders said they were determined to stand firm against a document they argue would devolve too much power to the regions and which demonises Saddam Hussein's Baath Party.
"We reject federalism in the central and southern regions, we reject it because it has no basis other than sectarianism," Adnan al-Dulaimi, head of an umbrella group called the National Conference for the Sunni People of Iraq, told reporters.
"Every Iraqi must stand in the way of all those who want to deepen sectarianism in Iraq."
In Hawija, north of Baghdad, hundreds of Sunnis marched in support of their political leaders and against the constitution, saying it had been presented to parliament without consensus.
Other small rallies were held in Baghdad and other cities.