The San Francisco Public Utilities Commission and the fire department have been called to shut off the water, police spokesman Albie Esparza said.
A police officer witnessed the broken main just after 9 a.m. this morning, he said.
The broken main is backing up traffic on the Interstate Highway 80 off-ramp to Fourth Street, according to the California Highway Patrol.
Two lanes of traffic on the off-ramp were closed as of 9:40 a.m., according to the CHP.
