Jan 19, 2007
It comes as coalition and Iraqi forces prepare for a full-scale crackdown against insurgents in the capital.
A spokesman for radical Shia cleric Moqtada Sadr said one of Mr Sadr's aides was arrested.
Iraq's Prime Minister Nouri Maliki is under increasing US pressure to dismantle Mr Sadr's Mehdi army, blamed for scores of sectarian killings.
The BBC's world affairs correspondent Mike Wooldridge says the Shia-dominated Iraqi government's willingness to take on Shia militia is widely seen as a litmus test of President Bush's new security plan.
It appears that operations against the militia are being stepped up, our correspondent says.
The US military says Iraqi special forces backed by US troops seized the as-yet unidentified man in a pre-dawn raid in eastern Baladiat district, close to the Mehdi army's stronghold of Sadr City.
It said the suspect was involved in the assassination of numerous Iraqi security forces members and government officials and the kidnapping, torture and murder of civilians.
Mr Sadr's office said its Baghdad media director, Sheikh Abdel Hadi al-Duraji, was arrested
"He was arrested at midnight [2100 GMT] with two cousins," Abdel Mehdi al-Matiri, a spokesman for Mr Sadr, told Reuters news agency.
"We are angry. This is a kind of revenge. Sheikh Duraji deals with the media. He is not a military man," he said.
The arrest comes a day after Mr Maliki said 400 members of the Mehdi army had been detained across southern Iraq.
But analysts say the prime minister is politically dependent on Mr Sadr's support, and some members of his party favour keeping him inside the political tent - rather than provoking him by trying to push him out.