"Some people smelled smoke, but we didn't hear anything and there was no damage that we were aware of," Cheesecake Factory host and server William Comer said.
Employees reported hearing an explosion in the building at 170 O'Farrell St. at about 12:30 p.m., said San Francisco fire Lt. Mindy Talmadge.
Firefighters located the fire in the roof area of the building and quickly extinguished it, Talmadge said.
Comer said some of the evacuated diners actually came back to their meals at the restaurant after the group was let back in around 1:15 p.m.
"Some people came right back in and others took the chance to bail on their meal," Comer said. "We basically all just hung out in Union Square until we were cleared to come on back."
The ruckus didn't stop the hungry lunch bunch from heading to the restaurant.
"We've been back for only 10 minutes and we already have a long wait list," Comer said.
In his two years at the Cheesecake Factory, Comer has never experienced an evacuation.
Joe Molica with Pacific Gas and Electric Co. said there have been no outages or hazard tags associated with an incident in downtown San Francisco.
A crew is en route to the scene to investigate the incident.