The blaze, located on the property of Trinity Christian Fellowship Church, formerly the site of First Christian Church of Castro Valley at 20409 Marshall St. was reported at 2:10 a.m., Alameda County fire spokeswoman Aisha Knowles said.
Firefighters arrived at the scene, a couple blocks north of Castro Valley Boulevard, to find a classroom on the church property on fire. The fire did not spread to other buildings and was knocked down within 20 minutes, Knowles said.
The classroom was not occupied at the time of the fire and no injuries were reported, according to Knowles.
The cause of the fire remains under investigation, but fire officials have determined that the blaze started on the exterior of the building, spread to the attic and then into the interior of the structure, Knowles said.