Parts of Silver Street, High Street and Hall Gate have all been cordoned off with people told to stay away from the scene.
Numerous eyewitnesses have reported dozens of police officers as well as a string of emergency vehicles and scenes of crime officers in attendance.
There have been unconfirmed reports of a stabbing.
One eyewitness said: “There’s police swarming everywhere, never seen so many.
"Something really bad must have happened. The streets are cordoned off and there’s loads of vans.”
A South Yorkshire Police spokesman said: “Please be aware that there is a police cordon in place in the area around Silver Street and High Street in Doncaster town centre this morning, and as a result, some buses are being diverted from the area. Please plan your routes accordingly, and avoid the area if you can.”