Another suicide bomber blew up a car bomb at a police checkpoint just south of the city, killing five and wounding 12.

The bombings were the worst in Iraq in at least six days, shattering a relative lull in the Sunni Arab insurgency against US forces and the Shia- and Kurdish-led government.

A senior Interior Ministry source said 20 people had been killed in the Baghdad blast and that the death toll could rise.

Twelve bodies lay under sheets surrounded by wailing relatives in a courtyard at the nearby Yarmuk hospital.

Doctors there said they were also treating 21 wounded, many in serious condition. Others may have been treated elsewhere and some bodies may have been collected by families at the scene.

The Interior Ministry source said the bomber wore an explosive vest beneath civilian clothes when he approached the ministry's special forces recruitment centre in the Mansour district of western Baghdad.

The same recruitment centre, near the Green Zone government and diplomatic compound, has been targeted by bombers several times in the past. An Interior Ministry source said recruits had been told to come on Saturday, normally a non-working day, in an effort to fox the insurgents and protect the volunteers.

"Apparently the precaution did not work," he said.

A senior police officer in Mahmudiya, just south of the capital, said police and civilians were among the five dead and 12 wounded when a suicide car bomber crashed into a police checkpoint in the centre of town.

Suicide bombings and car bombs have become the deadliest tactic in violence which has worsened sharply since the elected government took office in April. Police recruits are frequent targets, yet many Iraqi men continue to sign up in the hope of a paying job in a country where work is scarce.

Two police colonels were gunned down in their cars in separate incidents, one in the northern city of Mosul, another in Musayyib just south of Baghdad.

In a Shia Muslim neighbourhood of Baghdad, thousands of men held aloft the green-shrouded coffin of cleric Kamal al-Din al-Ghoureify, gunned down near his mosque as he drove to prayers on Friday.

Ghoureify was a Baghdad representative for Ayatollah Ali al -Sistani, recognised as spiritual leader by much of Iraq's Shia majority. Mourners packed the streets chanting and beating their chests, some brandishing AK-47 rifles, others holding portraits of the slain, turbaned cleric.

"It is a calamity for the neighbourhood, for Baghdad, for Muslims and for Shias," Ghoureify's weeping brother Abu Hussein told Reuters. "What was his guilt? He was an old man, 70 years old and paralysed. What did he do?"

Ghoureify's murder was one of three attacks on prominent Shia targets within 24 hours.

A suicide car bomber killed a bystander on Friday at a house in Baghdad used as an office by Prime Minister Ibrahim Jaafari, also a Shia. The previous night an uncle and a cousin of Shia national security adviser Mowaffaq al-Rubaie were shot dead in their shop in Baghdad with four others.

US Marines said they were checking to see whether they had killed a cousin of Iraq's ambassador to the United Nations in a raid last week.

Ambassador Samir Sumaidaie said Marines killed his first cousin's son, Mohammed al-Sumaidaie, an engineering student, during a June 25 raid on his home in Al-Shaikh Hadid, near a US military base at Haditha Dam.

"All indications point to a killing of an unarmed innocent civilian -- a cold-blooded murder," Sumaidaie, a Sunni Arab and US ally, said in a statement on Friday. "The Marines were smiling at each other as they were leaving."

The Marines said in a statement: "The events described in the allegations roughly correspond to an incident involving Coalition Forces on that day in that general location; therefore a military inquiry has been initiated to review the circumstances and the facts surrounding the incident."

Earlier a Swiss man of Iraqi origin was accidently shot dead by an American soldier in Baghdad, the Swiss press reported yesterday.

The death was confirmed by the foreign ministry in Bern, which declined to provide any other information concerning the incident.

"We are in contact with the appropriate authorities to find out the circumstances surrounding the death," said a spokesman for Swiss Foreign Minister Ivo Sieber.