Eye-witnesses described Lancaster as being “shrouded in thick white smoke” as a major blaze took hold at a gym on Thursday morning.
The smoke was visible across the city centre and south of the River Lune for several hours after the fire broke out at around 10am.
Shoppers covered their mouths as they escaped the acrid smoke.
Cordons were put in place across much of the central shopping area, with St Nicholas Arcades completely closed to shoppers, along with Lower Penny Street, Cheapside and part of Market Street.
The fire started on the second floor of a property in Penny Street which houses Inspire Gym.
Fire officers believe it had broken out in the gym’s sauna and quickly spread.
The roof collapsed shortly after firefighters had entered the building with breathing apparatus.
One man said he had seen the smoke spread across the city from close to Lancaster Royal Grammar School.
“I could see it from up the hill as it spread,” he said.
“It was incredibly thick and covered the whole of the city centre from what I could see.
“You couldn’t see any flames; it was just thick white smoke shrouding the whole city.”
Pensioner Margaret Baker had been sitting in a cafe in Cheapside when the fire broke out.
“The smoke was everywhere,” she said. “We were told we had to leave because the smoke was so thick and it set all the fire alarms off.
“I couldn’t believe how much smoke there was right through town.”
Tim Murrell, Lancashire Fire & Rescue’s general manager for the northern area, said the building’s roof caved in shortly after some of his crews had entered the property.
“The initial breathing apparatus teams had gone into the building to check everyone was out when the building collapsed from the roof down, so we had to get everyone out for their own safety.
“Because the roof had collapsed in on itself we asked for a building inspector to come out to look at the safety of the structure.” Mr Murrell said the teams will work into the night checking the building for ‘hotspots’ and smaller fires.
At its height crews were getting through 6,000 litres of water a minute in their bid to extinguish the blaze.
United Utilities were asked to increase the mains water pressure in Penny Street to assist with the operation.
Police cordons closed off much of the city centre for several hours.
Staff in shops were told to evacuate their customers but to then stay in the premises with windows closed.
Many businesses were able to reopen during the afternoon.
Mr Murrell said: “We made the decision to shut down the city centre because of the amount of smoke in the area.”
Aerial ladder teams from Morecambe and Preston joined eight other fire engines at the scene.
A command unit was also parked up in Market Square to help run the operation.
In total, around 50 firefighters from 10 crews across Lancaster, Morecambe, Milnthorpe and Preston were called into action.