The soldiers surrounded the Sunni al-Khayr mosque complex in the violence-plagued Khadhra neighborhood late Friday, said police 1st Lt. Thayer Mahmoud.
A witness said the troops burst in just after evening prayers. He and other worshippers were searched, questioned and photographed, and four of them were detained, he said on condition of anonymity for fear of reprisal.
Iraqi forces led the joint operation and discovered several rooftop fighting positions, assault rifles and firebombs, the U.S. military said in a statement.
Four other suspects were detained at other sites raided during the operation, the military said.
The eight are suspected of kidnapping, making car bombs, and financing and supporting terrorists, the military said.
They are believed to belong to an insurgent cell responsible for the killing of dozens of Narhwan area residents after the Feb. 22 bombing of a revered Shiite shrine in the central city of Samarra, the statement said.
"Authorities believe the atrocities were committed by terrorists attempting to provoke sectarian violence in the area," the statement said.
A large weapons cache was seized during the raid. It included 100 artillery shells, 100 rocket-propelled grenade warheads, 22 anti-tank missiles and several homemade bombs, the military said.