Amar al-Hakim, who was taken into custody on Friday, complained Saturday that U.S. soldiers handcuffed and blindfolded him before his release and "strongly abused" his bodyguards.

"Is this the way to deal with a national figure? This does not conform with Iraq's sovereignty," he said.

He said cell phones, licensed weapons and two-way radios were among items confiscated.

"It is not a question of offering apologies," the black-turbaned cleric told reporters in Najaf, about 100 miles south of Baghdad. "We need clear and honest measures to prevent such incidents from happening again."

Even though U.S. Ambassador Zalmay Khalilzad issued a rapid apology, the decision to hold al-Hakim and his complaints about his detention risk touching off a backlash from Shiite leaders at a time when their cooperation is needed most to keep a major security sweep through Baghdad from unraveling. At least 13 people died in attacks on Saturday in the capital, including five in a suicide car bombing at the headquarters of the political party headed by al-Hakim's father.

It also highlights the often knotty relationship between U.S. military authorities and Iraq's leaders, whose ties to neighboring patrons Syria backing Sunnis, and Iran acting as big brother to majority Shiites add fuel to sectarian rivalries and bring recriminations from Washington about alleged arms smuggling and outside interference.

Iraqi Prime Minister Nouri al-Maliki, a Shiite, on Saturday would only say the circumstances of the detention were still being investigated.

But Vice President Adil Abdul-Mahdi, a Shiite and a top leader in the parliamentary bloc controlled by al-Hakim's father, described the behavior of the U.S. troops involved in the detention as "inappropriate, foolish and haphazard."

The U.S. military said Saturday that al-Hakim was stopped in an area where smuggling between Iraq and Iran is common and detained after members of the convoy "did not cooperate with coalition forces and displayed suspicious activities." He was released to Iraqi authorities and his possessions were returned after further investigation, the military said.

"Mr. Hakim was treated with dignity and respect throughout the incident," the military said. "Unfortunate incidents such as this occasionally occur as Iraq endeavors to secure its borders."

About 8,000 people demonstrated near the Imam Ali mosque in Najaf, 100 miles south of Baghdad, against the detention, raising Iraqi flags and pictures of al-Hakim and his father.

"The detention of al-Hakim represents an insult to the Iraqi people," said Hassan al-Shebli, a 45-year-old storeowner who was among the protesters. "The Americans should avoid such irresponsible acts if they want to establish stability in the country."

Hundreds also took to the streets in Baghdad's main Shiite district of Sadr City and the southern Shiite cities of Karbala and Basra, but the protests were relatively small considering the influence of the al-Hakim family, indicating they were mainly aimed at sending a warning to the Americans.

"What happened is unacceptable," Shiite lawmaker Hamid Majid Moussa told Al-Forat television. "The Iraqi government and the American forces must put an end to such transgressions."

Al-Hakim's father, Abdul-Aziz al-Hakim, met with President Bush at the White House in December. He is the leader of the Supreme Council for the Islamic Revolution in Iraq, or SCIRI, the country's largest political force.