Police have asked parents to come forward with information after a large-scale disturbance in Cardiff city centre on Saturday night, involving local youths, left six people injured including some with stab wounds.
Four people have been arrested on suspicion of violent disorder after a huge fight broke out as bars in the Queen Street area of the city prepared to close at 10pm.
Witnesses described seeing people running for cover as two groups, some armed with weapons, clashed.
At least one person was said to be in hospital in a serious condition with head injuries following the disturbance.
Police said they believed the incident was "isolated" but that it was thought to involve local youths rather than drunken revellers.
A large part of the city centre remained cordoned off on Sunday morning as a team of 20 detectives scoured the area and attempted to piece together what had sparked the violence.
South Wales Police were also poring over hours of CCTV footage from across the city centre in an attempt to build up a clear picture of what had happened.
They have urged any parents who believe their children may have been involved in the violence to come forward so they can investigate the matter properly and prevent any further clashes.
Detective superintendent Esyr Jones from South Wales Police 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.
"This appears to have been an isolated incident involving local youths, all of whom can expect to find officers knocking at their doors as inquiries progress.
"We work closely with a wide range of partners to ensure that incidents of this nature are rare and the public should be assured that Cardiff remains a safe place in which to live, work and visit."
He added: "We are continuing to appeal to anyone who witnessed the incident or who has information which could assist the investigation to come forward.
"In particular, 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."
A taser was used to detain one man who police said had obstructed officers responding to the incident.
He is not believed to have been involved in the disorder.
Contact police via 101, quoting occurrence 2000424668. Or phone Crimestoppers anonymously on 0800 555 111.