Police are investigating two separate fights which broke out during the early hours of this morning in which three people were stabbed and a man was hit over the head with a bottle. Detectives believe the two fights are linked and possibly involve the same people.The first fight was reported to police at about 4am in Cambridge Street, and then at 5.53am officers received reports of another brawl in Carver Street, in which three people were stabbed and a man was hit over the head with a bottle. A South Yorkshire Police spokesman said: "Five people are confirmed as injured, with one person in a serious but stable condition in hospital. "Four people have been arrested and are now in police custody. Of these four, two have been arrested in connection with attempted murder, and two in connection with public order offences." South Yorkshire Police has now confirmed Barker's Pool, Cambridge Street, Division Street, Carver Street and Bethell Walk in Sheffield city centre have all now re-opened.Forensic officers have been examining the scene and were seen removing a white blood stained shirt from the area. Witnesses are urged to call 101. Alternatively call Crimestoppers anonymously on 0800 555111.