More than 120 firefighters are tackling a "massive" blaze on one of Glasgow's main shopping thoroughfares – Sauchiehall Street.
Residents are being told to close their doors because of the "suspected presence of asbestos".
The Scottish Fire and Rescue Service (SFRS) described it as an "extremely challenging incident" that would be "protracted".
Flames were seen in the roof of the building that houses Victoria's nightclub, and which also contains several shops.
Streets were cordoned off as thick smoke billowed into the air.
The occupants of nearby properties were evacuated, the SFRS said. Emergency services were called at 8.18am.
An onlooker who did not want to be named said it was a "massive blaze and it appears to be spreading. The amount of smoke is quite incredible".
General manager of the Pavilion Theatre, Iain Gordon, where firefighters were spotted gaining entry, said he was "very concerned".
The theatre – one of the city's oldest – has been there "for a long time and is a major part of Glasgow".
He added: "When you see how quickly the fire spread to other parts of the buildings in Sauchiehall Street we just hope the flames don't jump across the lane.
"It's a lane's distance but that's not very far when it's heat and flames going across."
No injuries were reported, but Scotland's First Minister, Nicola Sturgeon, said it appeared to be "extremely serious".
"This is a deeply concerning incident in the city of Glasgow," she said at First Minister's Questions.
Assistant Chief Officer Lewis Ramsay said: "This is an extremely challenging incident but the response and professionalism of our firefighters has been outstanding.
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"We would urge the occupants of surrounding buildings to remain indoors and keep their windows closed due to the suspected presence of asbestos.
"This will be a protracted incident and crews will remain on the scene as they work to prevent further fire spread and damage in this prominent city-centre location."
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