A 16-year-old boy is in Wellington Hospital in critical condition after afight among three people on the corner of Keith St and Glasgow St about 7o'clock on Wednesday night.
Whanganui police said the youth was admitted with head injuries to Wanganui Hospital but overnight Wednesday he was transferred to Wellington.
A 21-year-old involved in the altercation was arrested and charged with assault.
Police have yet to speak to the third person.
A number of bystanders who witnessed the altercation have spoken to police about what they saw take place.
Police are asking anyone who they have not yet spoken to and who can assist them with what happened, to call Detective Sergeant John Gleeson on 349 0612.