Detectives are unclear over the motives of two fights which resulted in five people being injured in Sheffield city centre.
Police are investigating two separate fights which broke out during the early hours of this morning in which three people were stabbed and two others injured.
Detectives believe the two fights are linked and possibly involve the same people and are urging witnesses to come forward with information.
Superintendent Caroline Rollitt said four men have been arrested - two on suspicion of attempted murder and two for public order offences.
The first fight was reported to police at about 4am in Cambridge Street and resulted in an Asian man suffering stab wounds.
At around 5.53am police officers received reports of a second brawl in Carver Street, in which three Polish men were stabbed. All three remain in hospital, where one remains in a 'serious but stable' condition.
A fifth man was hit over the head with a bottle during the second fight. He was taken to hospital with minor injuries and has since been released.
Supt Rollitt said officers were trawling through CCTV footage to try and establish exactly what had happened.
She said: "The first fight was a large brawl involving a number of people where an Asian male appears to have been set upon.
"Around 5.30am it appears that a second fight has occurred and when police officers attended they found three Polish men with stab wounds, one in quite a serious condition."
Officers arrested two men close to the scene on Carver Street, and two further men were arrested later in Spital Hill.
Police are unclear of the motives of the fights, but said they were not terror related.
Supt Rollitt said: "We do not know what the motives are for either fight yet.
"There has been a fracas between two groups of people, certainly in the first fight.
"I suspect there may be more people who have been injured who haven't come forward. Certainly one person turned up at hospital with injuries but has not wanted treatment.
"We don't know if the second fight is a continuation of the first or the three Polish men were just in the wrong place at the wrong time."
She added: "There will have been people who probably witnesses something, other people on nights out and taxi drivers, who aren't aware of what has happened.
"We want to speak to anyone who was in the area or who witnessed anything to come forward."
Extra police officers will be on patrol in the city centre tonight.
"These incidents are really unusual and we want to get to the bottom of it as quickly as possible," said Supt Rollitt.
"There will be extra patrols in the city centre tonight."
Witnesses are urged to call 101. Alternatively call Crimestoppers anonymously on 0800 555111.