Crews were on scene putting out hot spots flaring up Sunday morning.
The fire broke out at the building - which is under construction - around 6 p.m. Saturday evening.
The Berkeley Fire Department says the fire is still active and could burn through the end of the day or longer.
Nobody was inside - but neighboring apartment buildings were evacuated.
The fire chief says it could be awhile before they get inside.
"We had difficulty reaching the seat of the fire - it's burning near the center of the building and we're not sure of the stability so we can't send anybody inside," Berkeley Fire Chief David Brannigan said. "We've been here since 6 o'clock last night and we anticipate it could be as much as another 24 hours."
City building inspectors are checking for damage in nearby buildings. At least one three-story apartment building suffered water damage.
Nearly 100 firefighters battled a major six-alarm fire Saturday evening at a multi-story apartment building under construction in downtown Berkeley.
VIDEO: Crews battle 6-alarm building fire in Berkeley, officials say
The fire, in the 2000 block of University Avenue between Milvia Street and Shattuck Avenue, was reported shortly after 6 p.m.
Originally designated as a two-alarm fire, Berkeley firefighters eventually upgraded the incident to a six-alarm fire, receiving mutual aid from fire departments in neighboring Oakland, Piedmont, Emeryville, Albany and El Cerrito. The Alameda County Fire Department also assisted.
This is now a 6 alarm fire. We have no reported injuries at this time.— Berkeley Police (@berkeleypolice) November 22, 2020
#BREAKING Fire crews are on scene in Berkeley trying to put out a fire in what appears to be an apartment building under construction on University Ave near Shattuck. Neighboring businesses have been evacuated. pic.twitter.com/emDz9PxW5Y— Matt Boone (@MattBooneNews) November 22, 2020
Residents in neighboring apartment buildings were evacuated as a precaution.
Firefighters say, the foreman was on site earlier Saturday, but left at 5 p.m. No one was inside though the scaffolding and height of the building created challenges...
Keith May, Asst. Chief of the Berkeley Fire Department said, "It may not be safe to enter and get through that netting. We're assuming there's no sprinkler system active in there."
Much of the fire has been extinguished, but firefighters will remain on scene through the evening to extinguish hot spots.
Berkeley fire officials said none of the existing buildings ended up being damaged by the fire, though one apartment building nearby will have water damage and residents could be displaced.
There are no reports of injuries and the cause of the fire is unknown at this time and will be under investigation.
Officials say both directions of University Avenue remain closed between Milvia Street and Shattuck Avenue. The location of the fire is two blocks west of the University of California, Berkeley.
Picked up Postmates pizza and across the street is this inferno going on in Berkeley, CA. pic.twitter.com/ypVz9LlPuZ— TheVideoGameManiac (@TheVGM_SFW) November 22, 2020
Bay City News contributed to this report.