Four men have been arrested in connection with Friday's car bomb blast in Najaf which killed at least 95 people.
Iraqis protested in Baghdad, Basra and Najaf over the blast
The local governor said two of the suspects were members of the former regime from Basra, while the others were non-Iraqi Arabs subscribing to the puritanical Wahhabi Muslim faith.
The four men are said to have confessed to the bombing and to other plots intended to destabilise the country.
On Saturday an oil pipeline linking Iraq's oilfields in Kirkuk with the Baiji refinery to the south was on fire, after a possible sabotage attack, military
The arrests were announced as crowds gathered at the scene of the blast - outside one of the holiest Shia Muslim shrines - to prepare for the funerals of many of the victims.
Iraq's leading Shia Muslim politician, Ayatollah Mohammed Baqr al-Hakim, was among the victims.
BBC correspondent Valerie Jones, at the scene of the blast outside the Imam Ali Mosque, said the crowds were chanting against Saddam Hussein and the US forces.
"They are blaming Saddam Hussein supporters for the attack but also they are starting to rather vehemently blame the Americans for not providing them security," our correspondent said.
Mohammad Bahr al-Uloom, a Shia
dignitary on Iraq's Governing Council, has suspended his membership of the interim body in protest against
"This indifference [about protecting Shia holy sites] prompts
me to suspend my membership of the Governing Council, which was
unable to assume its responsibility of ensuring that coalition
forces protect our people, holy sites and religious authorities," he said.
The city's main hospital has received at least 95 bodies, and has treated hundreds of wounded.
Local officials say the attack was carried out with 700 kilograms (1550 pounds) of
explosives and hand grenades planted in two cars.
United Nations Secretary General Kofi Annan led calls for restraint among Iraq's political and religious groups.
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.
Iran and the US-appointed Iraqi Governing Council declared three days of mourning for Ayatollah Hakim.
The blast is the third major bombing in Iraq this month and comes just over a week after a blast at the United Nations headquarters in the city killed at least 23 people.
Mourners gathered in Najaf on Saturday carrying banners, flags and pictures of Ayatollah Hakim.
Ammar Abdel Aziz al-Hakim, one of his nephews, addressed the crowd.
"We have told the occupation forces that Iraq is for Iraqis and
not for them," he said.
In the southern Iraqi port of Basra, more than 5,000 people demonstrated against Ayatollah al-Hakim's death.
"The responsibility of Hakim's death lies with the British and
American forces because they neglected security," the marchers
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 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.
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)".