Multiple reports of a violent incident on Queen Street were received at around 9.50pm.
Six people have subsequently been taken to, or self-presented at, the University Hospital of Wales with various injuries.
One person suffered head injuries and remains in a serious condition, while three others are believed to have sustained stab wounds.
Two others have presented at the hospital with lesser injuries.
Two people have been arrested on suspicion of violent disorder and are currently in custody in Cardiff Bay.
An investigation is underway to establish the full circumstances, and anyone who witnessed the incident or events prior or after to it is urged to contact police via 101, quoting occurrence 2000424668.