A concrete block was thrown from the top of an 80ft (24m) multi-storey cark park in Devon, narrowly missing two people including a police community support officer (PCSO).
The 40kg block was thrown from the top of the multi-storey car park in Western Approach, Plymouth at 1700 BST on Wednesday.
Police said the block "narrowly missed" the PCSO and a member of the public.
A 16-year-old boy was arrested on suspicion of attempted assault.
Police have appealed for witnesses.