Four people have been arrested on suspicion of violent disorder after six individuals were injured in an alleged stabbing incident in Cardiff city centre, police have said.
One victim suffered head injuries and remains in a serious condition, while another three people are thought to have been stabbed, South Wales Police said.
The other victims suffered minor injuries in the "violent incident" on Queen Street.
All attended the University Hospital of Wales but two have subsequently been discharged.
Four people are in custody at Cardiff Bay police station under suspicion of violent disorder.
Police confirmed they received multiple reports of a violent incident at around 9.50pm.
Officers have described it as an "isolated incident involving local youths" and urged parents who suspect their child was involved to come forward.
Detective Superintendent Esyr 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.
"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 also 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.
In a statement, South Wales Police said: "An investigation is underway to establish the full circumstances, and anyone who witnessed the incident or events prior of after to it, is urged to contact police via 101, quoting occurrence 2000424668.
"Crimestoppers can also be contacted 100% anonymously on 0800 555 111."