Iraqi Shi'ite cleric Moqtada al-Sadr has urged his followers to stop battling government forces seeking to stamp down on violence in towns and cities that has threatened to spiral out of control.
"Because of the religious responsibility, and to stop Iraqi blood being shed ... we call for an end to armed appearances in Basra and all other provinces," he said in a statement given to journalists by his aides in the holy Shi'ite city of Najaf.
"Anyone carrying a weapon and targeting government institutions will not be one of us," the statement said.
A crackdown on Mr al Sadr's followers in the southern oil port of Basra has sparked an explosion of violence in Iraq's south and in the capital Baghdad.
Iraqi Prime Minister Nuri al-Maliki has ordered militants in Basra to lay down their arms and has extended a 72-hour deadline until April 8th.
The fighting has forced reluctant US and British forces back into action in support of a military operation launched last Tuesday by Mr Maliki against militants generally.
But so far only strongholds of Mr al Sadr's followers have been targeted and has sparked a furious backlash from his Mehdi Army militia.
Mr Al Sadr and Mr Maliki, who is supported by the Supreme Islamic Iraqi Council, are both Shi'ites who are in the majority in Iraq. The Mehdi believe the crackdown is an attempt to undermine them ahead of provincial elections due in October.
The US confirmed today that its forces were operating alongside Iraqi government troops in Basra. President George W Bush has called the crackdown a "defining moment in the history of a free Iraq".
But the spread of violence risks undoing a year of security improvements and jeopardising plans for US troops to withdraw. A six-month ceasefire ordered by Mr Al Sadr last year calmed matters significantly last year but while not formally called off it effectively as result of Mr Maliki ordering a general crackdown on militancy this year.
The fighting has established a new adversary for US forces, whose main military effort for the past year had been against Sunni Arab groups such as al Qaeda. Yesterday, Mr Maliki said the Mehdi was "worse than al Qaeda".
The fighting in Basra has spread to other towns throughout the Shi'ite south of the country and across Baghdad, especially the vast Sadr City slum, named for the Mr al Sadr's father.
In Baghdad, US forces have been involved in their heaviest fighting in the capital in months, with gunbattles and air strikes that they say have killed scores of fighters.
The capital has been placed under a curfew, which was extended last night indefinitely. All shops, businesses and schools are closed.