The bag was reported at 12:51 p.m. on Golden Gate Avenue just west of Larkin Street, San Francisco police Lt. Troy Dangerfield said.
A man in his 30s allegedly approached a U.S. marshal, pointed to the backpack and said it had explosive materials in it, and also made some anti-government statements, Dangerfield said.
The man left the scene and the marshal called 911, he said.
The Police Department's bomb squad responded to investigate the bag, and determined it only contained personal items, Dangerfield said.
People in the federal and state buildings on each side of Golden Gate Avenue between Polk and Larkin streets were asked to avoid exiting the buildings on the Golden Gate Avenue side during the investigation.
A message came over the intercom system in the federal courthouse building shortly before 3:30 p.m. saying, "The situation has been resolved, all operations are back to normal."
Several streets in the Civic Center area were also closed during the incident, including Golden Gate Avenue between Van Ness Avenue and Hyde Street, Polk Street between McAllister and Turk streets, and Larkin Street between McAllister and Turk streets, but were in the process of being reopened as of 3:30 p.m.
The man who allegedly made the statements about the backpack, described as a man in his 30s, was found nearby in United Nations Plaza and was identified by the marshal. He has been detained for questioning but has not been arrested, Dangerfield said.