Five post boxes and a public telephone were damaged by explosive devices in a two-hour Boxing Day spree.
The six explosions had a "significant potential to cause injury" but no-one was hurt, Sussex Police said.
The boxes and telephone kiosk were targeted by a saboteur on quiet roads near Chichester, West Sussex.
Police have told the public to "stay at least 50 metres away" and call 999 if they see anything suspicious at a post box.
The explosions took place between 22:00 GMT and midnight in Bosham, Lavant, Charlton, East Dean and Oving.
Det Insp Alan Pack said: "It's important to stress that we believe these criminal acts were intended to cause damage and nobody has been hurt, but there is significant potential to cause injury and we have dedicated officers to the investigation as a matter of urgency.
"Our crime scene investigators will be at the locations carrying out thorough examinations and officers will be carrying out house to house enquiries in the area."
Where the explosions happened
- Post box outside The Reach, Smugglers Lane, Bosham
- Post box at the entrance to Bosham Hoe on Hoe Lane junction with Smugglers Lane, Bosham
- Post box on the A286 junction with St Nicholas Road, Lavant
- Post box outside The Fox Goes Free pub, Charlton Road, Charlton
- Post box near the Star and Garter pub, East Dean
- Telephone kiosk at the High Street junction with Highfield Lane, Oving.