An air gun is thought to have been shot at a bus carrying passengers on the guided busway on Wednesday evening (October 10).
The incident happened around 6.30pm after the D bus left Parkside in Cambridge for Somersham.
A passenger on the bus, who did not wish to be named, said it was hit shortly before it reached Longstanton.
She said: "There was a really loud banging noise and the window started to crackle.
"They stopped the bus and told people to get off and said people might have targeted it with an air rifle."
She said that when she got off the bus, she could see an indentation in the outside of the vehicle below the window that had cracked.
She said she thought a passenger could have been hit if whatever damaged the bus had landed higher up.
"The bus drivers were in shock," the passenger added.
The passengers who had had to get off the bus were able to travel on one that was immediately behind it.
The bus travelling behind had been due to only travel to St Ives, but it continued on to Somersham so all passengers could reach their destinations.
Andy Campbell, MD of Stagecoach East, which runs the bus service, said: “We believe that someone fired an air gun at the bus, shattering a window, and so we had to take the vehicle out of service with immediate effect.
"We transferred all passengers to the bus that was following behind, which literally went the extra mile in taking customers as far as Somersham despite it being due to terminate at St Ives.
"Were proud of how both drivers handled this potentially difficult situation.”
The incident comes less then a fortnight after a ball bearing was shot at a lorry as it was travelling on the A14 at Fen Ditton.
At the time, police described the A14 incident as "despicable behaviour which could have led to a very serious or potentially fatal road collision".
A Cambridgeshire Constabulary spokeswoman said she could not find a record of the busway incident.