A teenager remains in a critical condition after he fell more than 15 metres from the roof of Melbourne's iconic Block Arcade building.

A Victoria Police spokesman said the 18-year-old man slipped from the roof of the Collins Street location about 3pm on Wednesday.

Along with two other people, the teenager took an internal lift to the building's top floor.

From there, the man managed to access the roof, but lost his footing while climbing onto a ledge.