The Alamo Plaza Shopping Center has been evacuated along with nearby businesses, and residents who live on Alamo Square Drive have been told to leave their homes immediately and go as far south as possible, according to the sheriff's office.
Residents have been advised to turn off all heat sources in their homes, lock all doors and windows, and take with them only essential items that can be carried. All pets must be on a carrier or a leash, emergency officials said.
Danville Boulevard is closed for about half a mile, between Las Trampas Road and St. Alphonsus Court, and the Stone Valley Road exit from Interstate Highway 680 is blocked off, San Ramon Valley Fire Protection District spokeswoman Kimberly French said.
Motorists are advised to expect delays and to avoid the area if possible.
The gas leak was reported at 3195 Danville Blvd. around 10:35 a.m. after an East Bay Municipal Utility District crew struck a 1.5-inch line with a backhoe, she said.
French said homes and businesses within a roughly 1,000-foot radius of the leak have been evacuated.
In addition to crews from San Ramon Valley Fire, responders include sheriff's deputies, PG&E, the California Highway Patrol, EBMUD and the East Bay Regional Park District, which is evacuating the Iron Horse Trail.
"All the folks here on scene are working together to formulate a plan," French said. "As soon as it is safe to reopen the area and let people back in we will do so," she said.
She said she did not have an estimate for when that would take place.