Several crews are tackling a blaze at a commercial building in Dereham.
The fire is thought to have started at Randells Garden Machinery, off Shipdham Road in Toftwood.
Residents said they heard "a loud bang" before seeing large plumes of smoke coming from the premises.
The fire service was first alerted to the blaze at 3.09pm, before sending its first appliance at 3.13pm and arriving at the scene four minutes later.
People living on Shipdham Road between Randells and the junction with Holland Court had initially been advised to evacuate their homes as a precaution.
They have now been told by firefighters and police officers that it is safe to return, but they must go inside straight away and shut all doors and windows.
Most have returned home, but some have chosen to remain at least a few hundred yards away.
Daisy Smith, who lives on nearby Hillcrest Avenue, was startled when she first saw the huge plumes of smoke.
"I was just sitting at home and I looked up to see these massive clouds of black smoke," said Miss Smith. "I really freaked out at first and called my family telling them to look outside.
"After that I just heard loads of sirens. I've never known anything like this to happen in Toftwood and I just hope whoever's involved is alright."
Police have blocked Shipdham Road at the junction with Hillcrest Avenue, while there is another police cordon at the junction with Holland Court.
Motorists looking to drive between Toftwood and Shipdham are currently being advised to seek alternative routes.
Smoke can still be seen billowing from the scene as firefighters attempt to battle the flames, their job made far more difficult by regular gusts of wind.
Just after 8pm, firefighters said the incident had been "scaled down", but that a smoke plume would be visible in the area for some time.
They said they hoped to partially open the road soon, but advised drivers to continue avoiding the area.
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