Scottish Gas Networks was called to the junction of St Vincent Street and Buchanan Street at around 3pm yesterday after people complained of a strong smell of gas near Nelson Mandela Place.
A large proportion of the road was fenced off and a hole dug to reach the pipes.
The repair work looks set to continue until this evening.
Marcus Christoforu, 37, who works in Buchanan Galleries, said yesterday: "They just came and planked a big fence in the middle of the road.
"It's not rush hour yet and it's already getting busy. I hope they are not working here for long and get it fixed soon."
St Vincent Street was cut to one lane were the fence was erected.
A spokesman for Glasgow City Council, said: "This is an incredibly sensitive site due to the impact of any works on city traffic.
"Where that is the case, we expect utilities to be working flat out to minimise disruption."
Scottish Gas Networks, which is responsible for gas repairs on Scotland's roads, is working to repair the leak.
A spokeswoman said: "We have closed a lane in St Vincent Street as well as part of Buchanan Street.
"We apologise for any disruptions which may be caused due to this and hope to have the work completed by this evening."