The protesters marched to the headquarters after a 5 p.m. rally in Frank Ogawa Plaza in support of Bradley Manning, a former U.S. Army intelligence analyst accused of leaking classified information to WikiLeaks.
The march of several dozen protesters arrived at the campaign headquarters about an hour later.
Police said that initial reports were that three protesters had entered the building, followed by reports that 60 protesters were inside. By 6:30 p.m., seven protesters were seated just beyond the front doors and refused to leave.
Several of the protesters sitting inside were wearing t-shirts reading "Iraq Veterans Against the War," including Scott Olsen, an Iraq War veteran seriously injured by a police projectile during an Occupy Oakland protest on Oct. 25.
Police later closed the doors to the building, leaving the larger crowd of protesters outside. Officers negotiated with the seven for several hours, and eventually arrested six of them for trespassing at the request for building management, according to police.
Olsen left voluntarily shortly before the arrests.
The campaign headquarters, located at Telegraph Avenue and 16th Street, had boarded windows when the march arrived. During a previous demonstration on Aug. 3, protesters smashed windows of the building while protesting at Oakland's monthly "Art Murmur" event.
The protesters said in a statement that they are demanding that President Obama apologize for statements they said he made regarding Manning's guilt, that the president ensures soldiers are free from pre-trial punishment, alleging that Manning was held in long periods of isolation, and that Manning be pardoned.
After leaving the headquarters, a smaller group of protesters marched to Oakland police headquarters.