Shia reaction to the detention was quick and sharp.
Hamid Majid Moussa, an Iraqi minister, told Al-Furat television: "What happened is unacceptable and an apology must be offered ... The Iraqi government and the American forces must put an end to such transgressions."
In Basra, about 300 SCIRI supporters protested against the detention.
|Ammar al-Hakim, who was detained by US|
forces on Friday, heads a charity [AFP]
"No, no to America," they chanted. "No, no to Satan."
Hameed Moalah, a Shia minister close to al-Hakim, said he was not sure what message Washington was trying to send, "but it is certainly a negative one".
Zalmay Khalilzad, the US ambassador to Iraq, issued a rapid apology.
He said: "I am sorry about the arrest. We don't know the circumstances of the arrest and we are investigating and we don't mean any disrespect to Abdul-Aziz al-Hakim or his family."
Al-Hakim's bloc carries the strongest voice in the 275-seat parliament.
It also maintains very close ties to Iran, which has hosted the elder al-Hakim and other SCIRI officials before the fall of Saddam Hussein in 2003.
To reinforce US worries about Iraq's stability, Jack Keane, a top Pentagon envoy and a retired army general, acknowledged that the "violence is too high" for Iraqi forces to handle alone.
US officials said four US soldiers were killed on Thursday in combat in Anbar province, but did not give specific locations or circumstances for the deaths.
"Has any of the Bush Iraq plans worked other than causing the worst destruction?"
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A US soldier was injured when a roadside bomb exploded in the southern Iraqi city of Diwaniya.
In the southern city of Basra, police said they arrested Issa Abdul-Razzaq Ahmed, a suspected Sunni fighter.
Mohammed al-Moussawi, a provincial police commander, said that Ahmed, 22, is on Iraqi interior ministry's most-wanted list, accused of financing and recruiting fighters.
Meanwhile, Sunni clerics used their Friday sermons to demand justice for two women who were allegedly raped by the mainly Shia led security forces.
Abu Hamza al-Muhajir, the self-proclaimed al-Qaeda leader in Iraq, has called on his followers to step up attacks on Iraqi security forces to avenge the rapes in Baghdad and the northern town of Tal Afar near the Syrian border.
"Go ahead with God's blessing and engulf their checkpoints in fire, destroy their homes, and spill their blood to flow as streams," he said in an audio tape released on the internet.
According to the Associated Press, at least 1,897 Iraqi civilians had been wounded as of Friday.
The actual number of casualties is believed to be far higher as many go unreported and dangerous conditions in Iraq make it difficult to collect and verify information.
In Ramadi, medical sources at a hospital said that 26 Iraqis, including four women and a child, were killed in a US air strike on one of the city's neighbourhood.
In Baghdad, Iraqi police found 14 unidentified bodies on Thursday.
In the northern city of Mosul, 10 bodies, one of an army captain, were found while another four bodies were found in Kirkuk.