Emergency crews were called to London Road after thieves stole copper piping and gas escaped into a building.
Police and firefighters were called to La Tosca Italian Restaurant, close to De Montfort Hall, at about 5.10pm following reports of a gas leak.
Engineers from National Grid were able to isolate the supply and cut off the gas by about 5.15pm.
Firefighters from central station then ventilated the building.
A spokesman for the fire service said the leak was caused by the theft of copper piping from the outside of the property.
Crew manager Steve Kisby, who was at the scene, said: "I thought it was a problem that was dying out, but this shows us that it's still out there.
"It's a danger to the public and to the people taking the copper pipes."
Noman Uddin, from the Akash Indian restaurant, a few doors away, said: "There were police outside La Tosca and we were told there was a gas leak, but it didn't seem too bad."
Crews left the scene by about 6pm.
