Halifax Fire Chief Doug Trussler said crews were working on a gas line when it was struck and ruptured. He said there is a two-inch gash in the gas line.
While the gas has been shut off, officials on the scene say it could take a few hours to purge the pipe and repair the hole.
Trussler said the evacuation of the area will likely last until about 1:30 p.m. AT.
All pedestrian and vehicle traffic is shut down in the area from Morris to Victoria Road and from Tower Road to Queen Street.
Officers are on scene directing traffic.
Peter Graham, spokesman for Capital Health, said both the IWK Health Centre and the Victoria General site are operating on emergency power, while the Halifax Infirmary site has been switched to a power feed from the north end of the city.
Graham also said surgeries scheduled at the VG are on hold for now.
There were reports that some people were stuck in elevators in a few buildings in the area.
'Really intense' hiss sound
Thompson Rahr, who lives on Fenwick Street, said he witnessed the leak.
"I saw a couple of paddy wagons show up, lights streaming, sirens going," he said. "The cops jump out and I’m looking around wondering what it was.
"It seems to be they were looking upwards where they’re doing natural gas work on South Park Street, right at the end of Fenwick. There was, in the background, kind of what sounded to be a fan coming out of, almost, a restaurant, like just a steady, strong hum that came along.
“It’s a really strong hiss, it almost compares exactly to a fan on the side of a restaurant, you know how they’re blowing that warmer air out of the kitchen or something like that? It’s just a very strong ‘hiss,’ almost but not as strong as a hiss, it’s just more hum — but really, really intense.”
Thompson also described what the leak looks like.
“I can see all of the dust that’s around the worksite. Basically it’s such a strong push that it’s blowing dust up in the air,” he said.