Firefighters responded to a call from a resident of the building, located near the corner of South King Road and Bowling Green Drive, at 1:18 a.m.
They found heavy smoke and flames coming from the building when they arrived, said Battalion Chief Scott Kouns.
Six people were rescued from the burning building, including one person who was on a balcony when firefighters arrived, ready to jump. Firefighters extended a ladder so he could climb down safely.
Smoke and flames were so thick at first that firefighters could only advance a fire hose about 15 feet into the two-story building.
Kouns said the common hallway through the building was thick with smoke and flames. "It's like a tunnel," he said. "Smoke and fire extended from one side to the other."
He said the fire was mostly under control after about 45 minutes, but that as of around 3:30 a.m. firefighters were still on the scene in "mop up mode."
"It took us a little while to get a handle on it just because there's so many apartments," Kouns said.
Kouns said 30-50 people were displaced by the fire, and that the cause has not yet been determined. He said most of the building was still intact, but a few units sustained heavy damage.