Firefighters received a call at about 11 a.m. about a fire at the Pavilion building in the 100 block of South First Street, fire Capt. Cleo Doss said.
After firefighters arrived, two men, one suffering from "very, very serious" burns and another from smoke inhalation, were assisted and transported to a hospital for treatment, Doss said.
The Santa Clara Valley Transportation Authority briefly suspended its light-rail service from San Fernando Street to San Carlos Street but soon resumed service, Doss said.
The one-alarm fire appeared to have originated in an electrical room in the below-ground garage. Firefighters shut down electricity to get to the room, Doss said.
The man who was burned was a maintenance worker at the building and the other man inhaled smoke while escorting the other man outside, according to Doss.
The fire was described as a "flash" that produced no flames afterwards, Doss said.
He said firefighters were still at the scene as of 12:30 p.m. and were trying to determine when it was safe to turn the electricity back on in the building.