Terrorists who carried out a fake attack on Meadowhall were "nullified" by armed police as part of a huge overnight training exercise.
A group of special firearm officers, playing the role of terrorists, stormed into the shopping centre armed with simulation explosives and firearms.
The terrorists then began shooting at shoppers as well as detonating explosives, triggering a police and emergency service response.
Members of security at Meadowhall began to quickly usher more than 850 volunteers, playing the role of members of the public, to the emergency exits.
Firearm officers in the area, wearing bullet proof vests and armed with simulation guns, then began to enter the shopping centre to deal with the terror attack.
Forces began to hem the fake terrorists into a designated area of Meadowhall to allow civilians to escape. However, the attackers then had to be "nullified" after it became apparent they would not surrender to officers.
Speaking after the training exercise, Deputy Chief Constable Mark Roberts, of South Yorkshire Police confirmed that mock "casualties and loss of life" were suffered during the exercise.
He said: "There were casualties and loss of life and that's why we had the emergency response teams on hand. The exercise was also to test out the ambulance service's response, not just the police's.
"If you have determined terrorists with explosives then there are going to be casualties and loss of life. That's why we need so much training, we need to be realistic because we are severe with our threat level.
"It's good to see how we would work with them and it's really important that we work together on a regular basis.
"It all ran to the timeline and we did not play it that way that we had everything we asked for straight there, we had a realistic time line.
"It was all played for real, there's no point doing it if they all had a script of what's happening."
Dep Chief Constable Roberts thanked the 857 volunteers that acted as members of the public, saying they helped make the exercise as realistic as possible.
He said: "We went to some effort and skill to make them up and make them look as realistic as possible and the response was impressive.
"However, it was also a test of the shopping centre as well as the emergency services to see how they would respond.
"It's a big undertaking by Meadowhall and all the emergency services. It's a cliche but we expect our staff to run towards danger in these situations so it's only right that we give those people all the training we can."