South Yorkshire Police has warned residents that the threat level of terrorism remains "severe" after carrying out a huge overnight training exercise at Meadowhall.
A group of special firearm officers, acting as terrorists, stormed into Meadowhall last night to carry out a fake terror attack.
The "attackers" set off simulation explosives and opened fire on more than 850 volunteers using simulation firearms to test the response of the emergency services.
Deputy Chief Constable Mark Roberts said there were mock "casualties and loss of life" suffered during the operation but confirmed the terrorists were later "nullified".
The exercise comes after similar ones carried out across the country, including at The Trafford Centre and on the River Thames.
The UK remains on a "severe" terror threat with ISIS carrying out a number of terrorist attacks in Paris, Brussels and Nice.
Dep Chief Constable Roberts said all the emergency services involved in last night's operation responded in the way he "hoped and thought" they would.
He added that it was vital the police took these "positive steps" to prepare for a terror attack and said he believes it is what the public would want.
"People in Sheffield should be reassured by this exercise. We are aware of the threat level and obviously we hope something like this never happens," he said.
"But we are taking positive steps which I believe is what the public would want us to do.
"We had police officers respond to the situation, as well as the fire and ambulance services, as events unfolded throughout the night and they responded to it as they would a real situation.
"Everyone responded in the way we'd hoped and thought they would to manage the situation."
Despite the terror alert being at "severe", police stressed that the terrorism exercise was not in response to a specific threat.
Dep Chief Constable Roberts said that the drill would be "fully debriefed" and any learning would feed through to "policies, plans and procedures for the future".
He said: "The original consultation with Meadowhall was over two years ago and this has built up and there has been a lot of effort put in place.
"The 'attackers' simulated explosive devices and simulated firearms. It does make quite a noise but I don't think people would have been very disturbed as there are not that many houses near Meadowhall.
"If anyone was affected then we are sorry about that but we needed to make it as realistic as possible.
"Our message is don't be alarmed but we all need to be aware without panicking. People need to be aware of the world we live in."