Calgary Mayor Naheed Nenshi says many businesses in the downtown core will not be able to return to normal operations after the Thanksgiving long weekend, following a fire in an underground electrical vault that has left thousands of customers without power.

Nenshi says Saturday's fire in the city's sewer system caused significant damage, and as a result, the affected electrical infrastructure will need to be rebuilt.

"This is going to take time," Nenshi told reporters Sunday afternoon. "We're not talking about a repair. We're talking about a rebuild of a large amount of infrastructure."

Approximately 5,000 customers have been left in the dark as a result of the underground blaze.

Enmax says several loud bangs were heard after the explosion. As a precaution, the utility company shut down power between 5th Street SW and 11th Street SW, and 4th Avenue SW and 7th Avenue SW to prevent further damage to the electrical system.

Nenshi says it could be Wednesday, at the earliest, before power is restored, and that the wait could be as long as next Saturday for some.

"It will not be possible early this week for there to be business as usual in those affected areas," Nenshi said. "There will be major disruptions to the affected area until the infrastructure is rebuilt."

Cause of fire under investigation

Calgary Fire Department Chief Ken Uzeloc said the cause of the fire is under investigation.

“It’s under investigation. Our priority was to get the fire out and ventilate it, so Enmax can take a look. It’s still too early to tell at this time,” Uzeloc said.

The fire occurred in a manhole at the intersection of 8th Avenue and 5th Street SW.

A large portion of Calgary’s power infrastructure was destroyed in the fire, which impacted 19 intersections, 112 residential and commercial buildings and thousands of homes.

While the air quality has been deemed safe by fire officials, Nenshi is urging people without power to temporarily leave of their affected homes.

“It’s important for people to stay warm and snuggle up,” Nenshi told reporters earlier on Sunday. “For now, we are looking to Calgarians to please look after your family, friends, as always, who are impacted by this, particularly on this Thanksgiving weekend.

“If you’re in a home impacted with electrical outage, unplug your electrical devices so there’s no power surge when the power goes back up,” Nenshi said, adding that during the Calgary floods, fire crews had put out fires due to the surge when people forgot to unplug appliances.

“We know it’s been a tough night for a lot of people and certainly a frightening night. We saw images last night no one was expecting to see. I’d like to thank Calgarians for their patience and thank them for looking after our friends and neighbours impacted without power,” Nenshi said.

Uzeloc said if safety systems -- such as fire alarms, sprinkler systems and emergency lighting -- are not operational, residents should not stay at home.

An Emergency Operation Centre has been opened at Mewata Armoury for affected residents to get information updates. The Calgary Emergency Management Agency is working with Enmax Energy and partners to assess the impact to residents and businesses.

The cost of the damages remains unknown at this time.