Police were called to a service bus in Sheffield after a passenger claimed she had been threatened with a knife.
The woman dialled 999 after a row broke out after she opened a window on the bus as it travelled along West Street, Sheffield city centre, on Tuesday evening.
Police officers boarded the bus to speak to passengers but the man accused of brandishing the weapon had got off.
He was last seen fleeing along Carver Street.
Anyone with information should call 101.