There has been widespread international condemnation of a bomb attack in the Iraqi holy city of Najaf which killed at least 80 people, including Iraq's leading Shia Muslim politician.
Some of Hakim's supporters called for revenge
Ayatollah Mohammed Baqr al-Hakim died and more than 100 people were wounded when a car bomb exploded outside the Imam Ali Mosque.
No group has claimed responsibility for the attack, one of the deadliest in the Middle East for 20 years.
United Nations Secretary General Kofi Annan appealed for calm in the wake of the attack, while US President George W Bush said America would help find the perpetrators.
Iran, where Ayatollah Hakim spent 20 years in exile, denounced the attack but said the US-led occupation forces were ultimately responsible because they had failed to maintain security in Iraq.
Shia figures suggest that supporters of ousted Iraqi leader Saddam Hussein could have caused the blast in an attempt to further destabilise the country.
It is the third major bombing in Baghdad this month and comes just over a week after a blast at the United Nations headquarters in the city killed at least 23 people.
Appeal for calm
Mr Annan's spokesman said the Secretary General condemned the attack "in the strongest possible terms".
"In the difficult days ahead," he said, "the Secretary General urges all political and religious groups in Iraq to refrain from further acts of violence and revenge".
Mr Bush said the attack was "a vicious act of terrorism... aimed at [Ayatollah Hakim], at one of Shia Islam's holiest sites, and at the hopes of the people of Iraq for freedom, peace and reconciliation".
The top US civilian official in Iraq, Paul Bremer, also denounced the attack, saying "the enemies of the new Iraq will stop at nothing".
Iran and the US-appointed Iraqi Governing Council declared three days of mourning for Ayatollah Hakim, while Russia called for a greater UN role in Iraq to help stop the escalating violence.
Scenes of panic
The blast happened as hundreds of people were leaving the mosque after Friday prayers.
The force of the explosion left a 1.5 metre (3.5 foot) crater in the street outside and destroyed at least two buildings across the road.
Part of the entrance to the mosque is said to have collapsed on the crowd, trapping many people.
Hours after the explosion, there was still chaos at the scene of the attack.
Outside the mosque, people screamed in grief and anger, while others searched through the twisted debris for missing relatives and friends.
Nearby, in the predominantly Shia Sadr City neighbourhood of Baghdad, about 1,000 supporters of Ayatollah Hakim held a demonstration and called for revenge.
Ayatollah Hakim was the leader of an Iran-backed group, the Supreme Council for the Islamic Revolution in Iraq (Sciri).
His nephew Mohsen Hakim told the BBC the bomb planted next to his uncle's car exploded as he got into his vehicle after leaving the mosque.
In his last sermon, the ayatollah had denounced loyalists of Saddam Hussein who he said were "now targeting the Marjiya (the top Shia religious leaders)".
BBC Middle Eastern affairs analyst Roger Hardy says that although the 63-year-old ayatollah was critical of the Americans, he was ready to work with them - a decision that earned him the hostility of more radical Shia factions.
But, our correspondent adds, if elements loyal to the old regime wanted to settle scores with the Shia and destabilise the country still further, they may have found a very effective way of doing so.
A Sciri spokesman in London, Hamid al-Bayati, told the BBC that when visiting Baghdad in May and June, he had told the US occupation authorities that protection of holy places and leading clerics should be stepped up.
"The allies did not respond to this proposal," Mr Bayati said. "I blame them for negligence in not protecting holy places and holy men."
A US military spokesman said no coalition forces were in the area "because
it is considered to be sacred ground".