The outage was reported at 9:51 a.m. and is affecting customers in San Leandro, Hayward, Castro Valley and San Lorenzo, PG&E spokeswoman Tamar Sarkissian said.
The cause remains under investigation and there is no estimate yet for when power will be restored, she said.
"We do apologize that so many customers are being impacted by this," Sarkissian said.
