Northern Territory Police are calling for witnesses to an assault in which a man sustained serious injuries in Alice Springs overnight.

A 30-year-old man sustained serious injuries to his arm after another man struck him with a weapon, near a taxi rank on Bath Street just after 9pm.

The offender fled the area and the victim flagged down a passing police vehicle, which stopped and provided assistance until St Johns Ambulance arrived on scene.

The man was transported to the Alice Springs Hospital in a serious condition.

Police arrested the offender, a 42-year-old man on Whittaker Street and he remains in police custody.

The area was declared a crime scene and remains closed this morning.

Police are calling for anyone who witnessed the incident or where the offender fled following the assault, to contact them on 131 444 or report it anonymously via Crime Stoppers on 1800 333 000 reference job 8958649.