A 17-year-old and 21-year-old were both killed while an 18-year-old was also injured.
A huge police cordon has been thrown up around the crime scene with a number of streets sealed off.
Police have closed off part of Hall Gate, a section of the High Street, Scot Lane and Silver Street to pedestrians while Sunny Bar, Market Place, Scot Lane, High Street, Printing Office Street and Priory Place are closed to motorists.
A 17-year-old is in custody in connection with the incident.
1. Doncaster double murder probe
Hall Gate has been cordoned off following the tragedy. Photo: JPIMedia
2. Doncaster double murder probe
A police car guards the scene in Silver Street. Photo: JPImedia
3. Doncaster double murder probe
The High Street has been closed off in front of the Mansion House. Photo: JPIMedia
4. Doncaster double murder probe
Cleveland Street has also been closed near to the Danum Hotel Photo: JPIMedia