Scenes of Crime officers have investigated the area at Monnow Street where a man was earlier found unconscious.
Police officers along with paramedics from the Welsh Ambulance Services attended the scene at 1.15am on Sunday morning, June 26 and a defibrillator was used on the victim before moving him to hospital.
Officers from the Criminal Investigation Department (CID) looked for clues at the scene as the victim was being treated in the University Hospital of Wales in Cardiff where his injuries are described as serious.
Monnow Street was closed between Agincourt Street and Lloyds Bank and along St John Street whilst investigations took place.
It is expected the street will reopen later today.
Anyone with details, including CCTV or dashcam footage, can call 101 or DM them, quoting log 2200213333. You can also contact Crimestoppers anonymously on 0800 555 111.