A group of the guard's colleagues were forced to surround her and pull her back from the victim while play continued on the field.
The A's opened an investigation into the incident overnight. The team already has suspended the guard's employer - a subcontractor hired by Staff Pro, the organisation in charge of security for A's games - for their game today, but no decision has been made on the group's status beyond then, according to the San Francisco Chronicle.
Witnesses told the newspaper the security guard appeared to go after the fan without reason and was booed when she returned to the section later during the game.
"We saw a line of about 5-6 security guards walking along the walkway on the top deck," one fan recalled, as reported by the East Bay Times. "The guard in front seemed to be looking for someone specific. They passed about 3 or 4 sections to our left, then returned."
The same fan said the guard returned to the scene only a few minutes later and had to be restrained again while she yelled at the fan.
"After a couple more minutes, police appeared," the fan added. "The guard returned, again; someone threw a half-empty beer at her and she responded violently, having to be held back before being pulled away from the scene one final time."
The A's commented on the situation following the game and have since referred the incident to the Oakland Police Department, a team spokesman told the Chronicle.
The guard who launched the attack has been barred from all future A's games, the spokesman added.
