Marin Avenue in Mill Valley was shut down between Bell Ln. and Flamingo Rd. following a major water main break Monday morning, a Southern Marin firefighter said.

The rupture was reported at about 10 a.m. outside of 235 Marin Ave. After assessing the scene, fire crews determined that the situation was more serious than initially believed, Southern Marin Fire Protection District firefighter Brett Stone said.

The 12-inch main was spewing about 1,000 gallons of water every minute, Stone said.

Firefighters attempted to contact the owners of the cars parked on the street and have them moved, fearing that a sinkhole could form, he said.

Marin Municipal Water District crews were called to the scene and shut off the water at about 10:20 a.m., Stone said.

Between 50 and 75 sandbags were filled and stacked to prevent flooding at one home. No other homes were threatened, Stone said.

The water from the ruptured main has lifted the asphalt, and it is unknown how stable the ground beneath it is, he said. No cars were being allowed on Marin Ave.

Stone did not know whether any residents had lost water service.