Armed police officers were seen near Decathalon in Eyre Street in Sheffield City Centre at around 11am this morning.
Multiple eye-witnesses have also reported seeing both plain-clothed officers and armed police in Abbeydale Road from 11.30am this morning.
A heavy police presence is still reported to be in place in the area, some four hours later.
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One eye-witness said they saw armed officers outside Sheffield Superstore on Abbeydale Road.
They said: “I heard lots of shouting and they had a man on the floor.
“There is still a heavy police presence, and at least one armed officer.
“My friend said they were searching a car on Belper Road, with gloves on.
“There were five cars, and it was evident later that there had been lots of plain-clothed officers around before the event.
“It was quite scary and intimidating, they were in full gear with guns. You don’t expect that.”
South Yorkshire Police have declined to comment at this time.