Six people have been taken to hospital following a large scale “violent disturbance” in Cardiff city centre.
Police and ambulance crews were called to Queen Street at about 21:50 GMT on Saturday.
One person suffered head injuries and is in a serious condition and three others are thought to have been stabbed, South Wales Police said.
Four people were arrested on suspicion of violent disorder. Det Supt Esyr Jones said it involved “local youths”.
On Sunday, police said three people remained at Cardiff’s University Hospital of Wales, with another at Llandough Hospital, Penarth.
None of the injuries are believed to life-threatening.
Police said a Taser was also used to detain one man who was obstructing officers, although he was not believed to have been involved in the disorder.
Officers were alerted by “multiple reports” of a “large disturbance” on Queen Street and an investigation is under way.
Det Supt Jones said: “I’d urge any parent who suspects their child was involved in any way to come forward.
“Tackling knife crime is the responsibility of us all and any parents who have suspicions about their children being involved are not protecting them by remaining quiet.”
Eyewitness Elizabeth Winter said: “Some groups of young lads seemed to be getting into altercations with the police. Then there was another man that was Tasered twice.”
Cardiff student Amogh George, 24, said he saw up to 30 police officers around Queen Street and the Capitol Shopping Centre after being alerted by “a lot of sirens”.
“People spoke about a brawl but I didn’t see what was happening,” said the journalism student.
Cordons remained in place on Queen Street on Sunday morning.
Det Supt Jones said: “This was a violent and completely unacceptable incident which has understandably caused alarm within the local community.
“A team of 20 detectives are currently investigating the incident, trawling through vast amounts of CCTV and speaking to a number of witnesses, and I want to reassure the public that we will be relentless in identifying and arresting all those involved.”