US troops detained on Friday the eldest son of Abdul-Aziz al-Hakim, one of Iraq's most powerful Shi'ite leaders, outraging Shi'ite politicians and prompting a swift apology from the US ambassador.
Ammar al-Hakim's convoy was stopped at a border checkpoint in eastern Wasit province as he returned from Iran, Iraqi officials said. The young cleric, who was taken to a nearby US military base, said he was handcuffed and blindfolded.
He was released on Friday evening after being held for most of the day, Iraqi officials said. One official said the soldiers had removed his cleric's black turban, an act Shi'ites would view as deeply insulting.
Ammar al-Hakim said US troops detained him because they said his passport had expired. He said the document was valid.
"The way I was arrested was disrespectful and not appropriate for a political and religious figure like me. They cuffed my hands and blindfolded my eyes. They raised their weapons against me," he told reporters after being released.
The incident could strain the previously good relations between Washington and Abdul-Aziz al-Hakim's Supreme Council for the Islamic Revolution in Iraq (SCIRI), the dominant political party in the Shi'ite-led government.
Washington has courted the elder Hakim in its efforts to stabilise Iraq and create a moderate Shi'ite bulwark against the growing influence of radical anti-American Shi'ite cleric Moqtada al-Sadr. The elder Hakim held talks with US President George W. Bush in Washington in December.
US ambassador Zalmay Khalilzad sought to contain any political fallout, saying while he did not know the circumstances of the arrest "we do not mean any disrespect" to the family.
"I am sorry about the arrest. We will find out what happened and we will let Mr Hakim and the Iraqi government know," he told reporters. The US military had no comment.
US embassy spokesman Lou Fintor said Ammar al-Hakim had been arrested by soldiers "doing their duty at a checkpoint".
"He was not singled out. We understand soldiers were following standard procedure because the border was closed and once the facts of the situation were determined, Mr. Hakim was released. We are continuing to look into the situation," Fintor said.
Iraqi security officials said US forces took Ammar al-Hakim to Forward Operating Base Delta, a US camp near Kut, capital of Wasit province.
Shi'ite officials called the arrest a "cowboy" action and said it could spark street protests. The provincial council of the southern Shi'ite province of Najaf called for demonstrations on Saturday.
"The way he has been treated is unacceptable and it will definitely provoke many people," said Hameed Mualla, a senior SCIRI lawmaker.
Education Minister Khodair al-Khozaei, a Shi'ite, condemned the detention.
"There will definitely be popular reactions," he told Dubai-based Al Arabiya television.
"More than anybody else, it is the Americans who are violating the security plan through their cowboy methods," he said, referring to a major security crackdown launched this month in Baghdad.
Ammar al-Hakim, in his mid-30s and normally dressed in black turban and clerical robes, is an influential member of the ruling Shi'ite Alliance in his own right and is secretary-general of a humanitarian charity set up by SCIRI.
The Washington Post reported in August 2005 that he went to Washington to meet officials at the State Department, Pentagon and National Security Council.
US forces have in recent months arrested a number of Iranians in Iraq, including several who were seized at Abdul-Aziz al-Hakim's compound in Baghdad. They were later released.
SCIRI was founded in Iran in 1982 and its continued links to Shi'ite Islamist Iran are a source of concern for Washington, which accuses Iran of fuelling violence in Iraq by supplying Shi'ite militias with weapons, funding and training.
Its military wing, the Badr Organisation, fought with Iran in the 1980-88 war against Saddam. Sunni leaders accuse it of targeting members of the minority sect, a charge the elder Hakim denies.