Police are searching a disused brewery after a man carrying what is believed to be a gun was seen entering the building.
South Yorkshire Police said there is a large police presence in the Neepsend Lane area of Sheffield after reports about the man were received at about 5.15am on Friday.
A spokesman said: "Officers were immediately deployed and secured the scene.
"The area being searched for the man within the brewery and disused land is vast and to ensure the public’s safety while this is carried out, a number of roads have been cordoned off and will remain closed throughout the morning."