The cast iron pipe, installed in 1935, broke at about 1:30 p.m. on Monticello Street just off of 19th Avenue, SFPUC spokeswoman Alison Kastama said.
Water flooded into a couple of homes on nearby Chester Avenue before the flow of water was shut off, Kastama said.
Nineteenth Avenue remained closed in the area as of shortly before 4 p.m. while crews work to clear sediment that came out along with the water and is affecting San Francisco Municipal Railway's M-Ocean View line, Kastama said.
Buses are taking Muni passengers around the affected portion of the light-rail line, she said.
A few isolated water outages have also been reported along 19th Avenue, according to Kastama.
There was no estimate yet for when the roadway will reopen, Kastama said.