The fire was reported about 9:40 a.m. in the basement of the Sonora School of Sewing.
Officials say it was confined to the basement area, and they do not yet know the cause of the fire or the extent of damage.
The fire was extinguished about 10 a.m., according to Sonora Fire Chief Mike Barrows.
A wooden gate in the basement that separated the basement from a drainage grate on the sidewalk of West Dodge Street caught fire.
A cigarette thrown into the drainage grate is the suspected cause.
The only damage appears to be to the wooden gate. No water was used in the basement, as firefighters pulled out the burning wood and the fire stopped.
Southbound traffic was diverted from North Washington Street to Stewart Street.Both lanes are now open.