A Royal Bank of Canada office building was evacuated and King Street West is closed after an explosion caused by a hydro vault fire underground led to heavy smoke in the financial district on Monday afternoon.

Plumes of brown smoke were seen rising from a sewer grate along King Street West between Bay and Yonge streets shortly after 5 p.m. as commuters were leaving work, Toronto police said. 

The smoke was caused by a blown out hydro vault fire, Toronto Fire Service Chief Matthew Pegg confirmed, that had been contained and controlled. Crews were dealing with smoke migration, he said.

These vaults contain high-voltage transformers, said Toronto Hydro spokesperson Brian Buchan. This type of incident is possible because many of these vaults and other electrical equipment is stored in the underground area near Scotiabank Plaza, he added.

In an effort to contain the smoke, fire crews brought in CO2 extinguishers and evacuated a Royal Bank office tower as a precaution, along with parts of the underground shopping concourse. 

"Avoid the area, don't breathe in the smoke that is admitting from there," Ratushniak said.

The smoke contains carcinogens which are found in an electrical fire, he added.

While Toronto Fire hasn't determined what exactly caused the blow out, Platoon Chief Kevin Shaw told reporters on Monday evening that "typically rain, humidity, dampness seems to get into the infrastructure that may be a little bit older and deteriorate. They cause these fires."

Witnesses who were leaving work said they heard several loud bangs which sounded like explosions. 

Police said no injuries had been reported.

PATH businesses are also closed from Queen to Front streets, Toronto Fire said.

King Street is closed between Victoria and Bay streets, and police have since closed off Wellington Street to Adelaide Street.

Subway service is bypassing King station both ways on Line 1 due to an investigation on street level, TTC spokesperson Brad Ross told CP24.

The 504 and 514 streetcar routes are also being diverted along Queen Street West between Spadina Avenue and Church Street.

Shaw said the area will remain blocked off until the hydro vault is de-energized and the fire is fully extinguished.