The Iraqi news agency said security forces had made several arrests and were seeking other people suspected of involvement in the killing of Grand Ayatollah Muhammad Sadiq al-Sadr.

An Iraqi opposition group based in Syria reported that riots broke out in Baghdad and across southern Iraq following the assassination.

Grand Ayatollah Muhammad Sadiq al-Sadr was the prayer leader at the Iman Ali Mosque in the Shi'a holy city of Najaf, 150km south of Baghdad.

The BBC's George Alagiah: "It took Baghdad 36 hours to announce the killing"

The French news agency AFP quoted witnesses in Najaf who described violent confrontation between the ayatollah's followers and security forces in which several people were killed.

Reports from Baghdad say the authorities sealed off the mainly-Shi'a Saddam City suburb after clashes between Shi'as and the security forces.

Saddam's visit to Najaf in March 1998: His first for many years

Shi'a organisations in London said Mr Sadr was put under house arrest two weeks ago after he started criticising the government during Friday prayers.

The sources said he recently sent a letter to Shi'as outside Iraq asking them to work to protect him from government harassment.

Two leading clerics were assassinated in 1998 and international Shi'a groups have accused the Baghdad Government of having a hand in the killings to crush the independence of clerics.

Well known figure

Mr Sadr was a prominent Shi'a figure in Iraq who had periodically clashed with the government of Saddam Hussein.

He is said to have supported the uprising of Iraqi Shi'as in the wake of the Gulf war in 1991 but was later reconciled with the authorities.

He was Baghdad's choice to succeed Ayatollah Al-Khoei, who died under house arrest in 1992, as the supreme authority of the Shi'a faith in Iraq.

Mr Sadr's Friday sermons at the Imam Ali Mosque in Najaf had been broadcast on Iraqi TV.

However, there are reports that recently he had begun to criticise Iraqi Government for the strong security presence in Najaf to police Shi'as.

Shi'a sources say that the Ayatollah knew his life was in danger and had sent some members of his family abroad.