A teenager has been arrested for allegedly terrorising Melbourne motorists in a fake emergency services truck he modified himself.

Detectives from the Heavy Vehicle Crime Investigation Unit allege a 16-year-old Brighton teenager fitted the truck out with red and blue flashing lights, a spotlight on the rear of the vehicle, and even installed an emergency siren.

Police became aware of the bogus emergency truck after seeing online footage of the truck weaving through traffic with emergency lights flashing.

Detectives allege the truck was modified so convincingly, frightened motorists were pulling over to the side of the road as it sped past with its siren blaring.