TORONTO (Reuters) - Explosions from a transformer fire rocked Toronto’s financial district for hours on Monday afternoon, prompting police to close off a section of the downtown area as heavy smoke billowed up from subway grates.

The blast shut down several blocks along King and Bay streets during evening rush hour but a Toronto police spokeswoman said there were no injuries reported, and police and fire trucks were on the scene.

A fire in a transformer in an underground electrical vault caused thundering explosions that continued intermittently for hours, Toronto Fire Captain Adrian Ratushniak told Reuters.

The cause of the fire, which prompted the evacuation of a bank tower at 20 King St. West, was not yet known and would be investigated, Ratushniak added.

Previous, similar fires have been caused by water, deteriorating electrical connections or aging transformers, he said.

The clouds of smoke that filled the block and adjacent intersections were likely caused by burning plastic or PVC insulation, Ratushniak said, and the explosions the result of electrical wiring “touching something it’s not supposed to.”

Toronto Fire arrived on the scene just after 5 p.m. and two hours later, some 40 officers remained battling the fire, which had not yet been confirmed extinguished but was largely under control, Ratushniak said.

“Hydro vaults (electrical transformers) are relatively contained” because of the concrete boxes encasing them, he said. “You don’t typically see flames visible.”

Subways were bypassing King station while streetcars were being diverted from the area.