Several stores and restaurants at a downtown shopping centre in Montreal were closed Thursday after a massive fire evacuation.

The fire department believes a grease buildup may have sparked the blaze at the Complexe Desjardins on Wednesday night. (CBC)

More than 5,000 people were forced to leave Complexe Desjardins on Wednesday night after a fire in the ventilation system of the Baton Rouge restaurant on Ste-Catherine Street.

The cleanup operation is underway, but assessing the damages will take a while, and it's not clear when stores can reopen, owners affected by the fire said.

"They will use a disinfectant to take away the smell" of smoke and mould, said Louise Audet, who manages the Graciane clothing store.

The damages could destroy business, said Athanasios Argiriou, a furrier next door to Graciane.

"Touch here, touch here, it's water … it's water … now the water gets into [the leather]," he told CBC News, as he showed off a giant hole in the ceiling of his store, where firefighters broke through to reach a vent.

The fire forced more than 5,000 people out of the Montreal shopping centre. (CBC)

Argiriou estimates the fire ruined as many as 75 fox, mink and beaver coats, worth at least $250,000.

The fire put out hundreds of guests at the Hyatt Hotel, which is attached to the shopping centre.

Nearly 240 guests had to be relocated, and "nobody is back officially into the hotel," said manager Patrick Cappa. "They're allowed to come into the hotel, and obviously, pick up their belongings. But we have not opened the hotel yet."

The fire department believes a grease buildup in the ventilation system above the Baton Rouge may have sparked the fire.