The break, which officials say was caused by a construction crew working near Tomaso Court and Sunnydale Avenue, began at 11 a.m. Crews are still working to shut off the water.
Nearby streets and properties are flooded, including Our Lady of the Visitacion School. Students have been told to shelter-in-place and school officials say the damage from the water is already so bad that the school will be closed for several days to make repairs.
Bayshore Boulevard is closed at Sunnydale Avenue and motorists are being asked to avoid the area. Muni service has been impacted; the No. 9 San Bruno bus will not be making stops along Sunnydale Avenue between Cora Street and Bayshore Boulevard.
The flooding has spread into nearby Daly City so officials from that city are also responding.
Water service has been impacted for Visitacion Valley, Bayview, Ingleside and some Daly City residents.