A "strong gas smell" was reported in Bridgnorth just before 2pm, and the emergency services have been telling people to evacuate their premises.
Three fire crews attended from Bridgnorth and Telford.
The Shropshire Fire and Rescue Service said that the electricity board had also attended and isolated the area's power supply.
The leak was reported to have been traced to the former NatWest building.
The area remained cordoned off at 8.10pm tonight, with emergency services unable to say when the cordon might be lifted.
Shropshire Fire and Rescue Service says the leak has been isolated.
A message posted on the Love Bridgnorth Facebook page tonight read: "The good news is the gas leak has been found. The bad news is it’s massive.
"We must all be extremely grateful that it was discovered before anything awful happened.
"Sorry folks the police and firefighters reckon it’s going to be a long time before the evacuation zone is lifted and the area may even be extended.
"If anyone is aware of any vulnerable people in the area please let us know - there are local residents and police around who can help."
Just before 8.50pm tonight, West Mercia Police said officers had reduced the cordon and the majority of residents have been able to return home.
Western Power engineers are currently working to restore electricity that is off in part of the High Street.
A small cordon will remain in place overnight and several residents are being housed for the evening by Bridgnorth Town Council.
Superintendent Jon Marsden said: "The gas leak in Bridgnorth High Street has been isolated, the cordon has been scaled down and the majority of residents from the area are now able to return home.
"Those that aren't able to return to their homes are being kept informed of developments and have been housed for the evening by Bridgnorth Town Council.
"I am pleased to say there have been no casualties during the incident and I would like to thank the local residents for their patience and understanding the need for us to secure the leak and make the area safe.
"This has involved many agencies from across the area including local authority, fire service, NHS, gas and electricity companies. I would like to thank all those involved for pulling together to respond quickly to the incident."
Bridgnorth high street remains closed while specialist gas engineers carry out further work.— Broseley&Wenlock SNT (@WenlockCops) May 20, 2019
I've just got back from Bridgnorth. There was a strong smell of gas on the high street. The road has been closed and the immediate area has been evacuated.— Claire Mcilvenna (@clairemcilvenna) May 20, 2019