A man was threatened with a knife and tied up in front of other passengers after he told off a smoker.
The victim's hands were bound behind his back by the knifeman on the E bus service at St Thomas railway station in Exeter, Devon, on Saturday lunchtime.
The offender wielded a 3-4in (7-10cm) long knife to threaten the passenger after the man scolded him for smoking.
He taunted the victim by banging on the window after he had got off before walking to Exe Bridges Retail Park.
Police said a 23-year-old Exeter man was arrested and released on bail.
The victim was not physically injured but described as in shock.
Officer have appealed for other passengers on board the bus who witnessed the incident to come forward.