The break on the 1600 block of Parker Street was reported at 4:08 p.m., according to Deputy Fire Chief Avery Webb.
Water from the break between McGee Avenue and California Street was flowing west and flooding the intersection of California and Parker streets, Webb said.
The water reached as far as Sacramento Street, where it flowed into the underground storm drain, and overflowed some curbs into yards in the area, Webb said.
"It was threatening homes," Webb said.
Firefighters requested sand bags from public works crews to protect homes in the area, but were able to shut down one valve before they arrived, slowing the flow of water, Webb said.
East Bay Municipal Utilities District crews arrived on scene around 4:42 p.m., Webb said.
EBMUD spokeswoman Andrea Pook said around 35 customers on the 1600 block of Parker lost water service due to the break.
The location of the break made access difficult, but work crews were able to shut off the water flow shortly after 6 p.m. and repairs are underway, Pook said.
The cause of the break remains undetermined, Pook said.