The official from the Popular Mobilization Forces said on Friday that Aws al-Khafaji, commander of the Abu al-Fadl al-Abbas Brigade, was detained the previous night.
The official spoke to The Associated Press on condition of anonymity because he was not authorized to talk to reporters.
Al-Khafaji had earlier told a local TV station that Iraqis should not only oppose American and Turkish presence in Iraq but also Iran's interference.
A statement from the Popular Mobilization Forces says four Baghdad offices of al-Khafaji's group were closed because they were located in residential neighborhoods. It said members were arrested after rejecting the closure.