The break occurred at approximately 8:40 a.m. and San Francisco Public Utilities Commission crews on the scene isolated the break and shut off water to the breached pipe.
The flooding is coming from a high-pressure fire service water main and occurred during pressure testing, according to the SFPUC. The pipe was used for firefighting and emergencies, so the break is not expected to impact nearby residences or businesses' drinking water. Two nearby fire hydrants are out of service.
Muni's 9 and 27 bus lines are experiencing delays and are being rerouted.
Repairs are underway, according to SFPUC spokeswoman Jean Walsh, and may take up to one week.
The burst cast iron pipe flooded the street, which remains closed to traffic. The pipe dates back to the early 1900s, when the emergency water system was first constructed in response to the 1906 earthquake and subsequent fire.
The system is being upgraded as part of two bond measures passed by San Francisco voters in 2010 and in an election this month.
Bay City News contributed to this report.