The fire, which has now been extinguished, broke out in the early hours of Monday morning and tore through The Grasshopper Building in Whitmore Reans.
Residents described hearing crackling and feeling an immense heat from the blaze which was reported just before 1am.
Shasta and Zainab were in a house directly behind the warehouse while it was ablaze and had to evacuate in the middle of the night.
Zainab said: "By the time I was called it was already in the air blazing.
"Initially we were told to stay indoors and close our windows at around 1am.
"We started hearing crackles at sharp noises so we ran downstairs and all we could see was complete fire, it was mental.
"One of the neighbours was running around banging everyone's doors telling everyone to leve their houses.
"We could feel the heat in the front room, the fire officers said it was good that we evacuated because we would have been told to anyway.
"They told us it was safe to go back at around 2.30am, the kids haven't been to school because we were up so late.
"It was a nightmare, we were told if we hadn't moved our cars the windows could've smashed from the heat."
Thirteen fire engines and between 70 and 80 firefighters were required tackle the blaze at the building just off Craddock Street and Gatis Street.
The fire service described the blaze as 'a difficult one to tackle' but they did get on top of it scale it down at around 3.30am.
Yusuf Malik, who lives a couple streets down from the warehouse said: "Just after midnight my wife spotted the fire from the bedroom.
"Within 20 minutes it had progress quite vigorously, she called the ambulance and fire brigade.
"When I arrived the full length of the building was alight.
"As the fire brigade got there all the walls had collapsed, we heard them drop, there was a lot of people around watching on."
Police and paramedics were also sent to the scene but nobody is thought to have been hurt.
The steel-framed warehouse includes offices and belonged to the Grasshopper baby clothing company before becoming an Xercise4Less gym in 2014.
A family who live next to the factory have had to stay at home with their children after being out of the house until 3am.
They said: "We came home at about 11pm and my daughter said she could smell fire and burning but we couldn't see anything.
"Then at around 1am we were just getting into bed and we could here popping which we thought was weird.
"My daughter then came in and said 'mom the factory is alight.
"The kid's haven't been to school because we were up so late."
The building has been left gutted and investigations into the cause of the blaze are now ongoing.
An elderly resident explained how she started hearing what she thought were fireworks at around 1pm.
She said: "I looked through the blinds and for a split second thought 'why would somebody light a bonfire'?
"Then god, it was just a full blaze, it was so shocking."