Power has been restored to 35,000 PG&E customers in downtown Walnut Creek and surrounding communities.
The homes and businesses lost electrical power shortly after 9 a.m. today.
PG&E spokeswoman Tamar Sarkissian said the cause is under investigation. She said the company expects the investigation to take awhile.
She said the problem was centered at the Lakewood substation on Ygnacio Valley Road near A quartet of PG&E trucks were there Tuesday morning and red and yellow tape was drawn around two large portions of the utility yard.
Sarkissian said all affected customers had power back by 10 a.m.
She said 29,000 customers were from Walnut Creek. There were 2,000 each from Alamo, Pleasant Hill and Lafayette.
"We apologize for this inconvenience," said Sarkissian. "We know it's difficult when power goes out."
A dispatcher at Walnut Creek police said the outage appeared to affect about half the downtown core as well as surrounding areas and parts of Ygnacio Valley Road.
The police department's power went out briefly before back-up generators took over. The dispatcher said officers had to help a couple people out of an elevator that became stuck.
She said police also received calls about people being trapped in elevators in other parts of the downtown region.
Sgt. Tom Cashion said no traffic accidents were reported despite the fact a number of stoplights in the area were without power. Cashion reminded motorists that an out-of-service stoplight should be treated as a four-way stop.
Students at Las Lomas High School said the power went out there about 9 a.m. but was restored in about a half-hour.