A collapsed storm sewer that flooded an underground transformer vault is being blamed for explosions and power outages in downtown Saskatoon Monday afternoon.

Hundreds of people were evacuated from residences at the Second Avenue Lofts and an adjacent office building shortly after 2:30 p.m. when a series of explosions under the sidewalk in the 200 block of Second Avenue North spewed black smoke into the air.

Investigators have traced the source of the incident to a collapsed storm sewer pipe underneath a catch basin that resulted in the transformer vault being flooded with water.

Saskatoon Light and Power says the pump within the vault was working, but it couldn't discharge the water as the catch basin was not functioning properly.

A resulting power failure in the downtown area saw business close their doors and thousands of people were asked to evacuate downtown buildings, including Midtown Plaza.

City crews were able to restore power to all affected areas before midnight Monday.

-with Canadian Press Files