The outage was reported at about 3:45 p.m., spokeswoman Fiona Chan said.
Chan said the outage was caused by a tree that came into contact with power lines in the area of Clarendon Avenue and Oak Park Drive.
PG&E had restored power to about 1,600 customers by 4:30 p.m. and were working to get power back to the rest, she said.
She advised anyone who sees a downed power line to stay away from it and call 911.
“When they see downed power lines, they should assume they are live,” Chan said.