South Yorkshire Police said earlier today that the officer was injured in Cherry Tree Lane, Hexthorpe, after the occupants of a car which crashed in nearby Cleveland Street, fled.
But extra details have been released this afternoon, with the South Yorkshire branch of the Police Federation revealing that the officer was responding to a report of an armed robbery at the time of the incident.
The officer was rushed to hospital with serious injuries as he tried to make an arrest.
Police Federation chairman Zuleika Payne said: "Sadly we learn that one of our officers has been attacked whilst responding to a report of an armed robbery.
"The officer has sustained serious injury and immediately been conveyed to hospital.
"When police officers commence a tour of duty, they never know what they may face. Again, this shows the determination and courage of our officers who put themselves in harm's way in order to protect the public.
"The thin blue line is being stretched even further, we have to protect officer numbers. Assaults on officers should not be part of the job."
Two men and a woman have been arrested.