Damage to the water main on the Pleasanton Ridge was caused by a landslide triggered by recent heavy rains, according to Daniel Smith, the city's director of operation services.
Crews are currently working to repair the main, and work is expected to last through the weekend or possibly longer, Smith said.
The repairs could take longer than expected because of the "challenging and steep terrain," Smith said.
Most of the affected homes, which have little or no potable water, are located in unincorporated Alameda County, with a small portion of residents who live in Pleasanton affected, he said.
The outage affects residents who receive a water bill from the city of Pleasanton in upper Kilkare Canyon, which stretches from Pleasanton Ridge to Sunol, Smith said.
Several homes on upper Longview Drive have also been affected, along with a number of homes at the end of Santos Ranch Road, he said.
Residents are advised to discontinue water use until Sunday, although Smith said tap water in the affected area has not been deemed contaminated. Test results regarding the water's safety are expected on Sunday, he said.