Police in South Yorkshire said it "was a miracle no-one was hurt" after stones and other debris were hurled at passing buses from a bridge.
Officers said youths had been throwing stones and other objects at the buses in Sheffield Road and Bridge Street in the Killamarsh area of Sheffield.
Three buses suffered damage in the spate of attacks, in two instances the vehicles' windscreens were smashed.
Passengers on another bus escaped uninjured when objects were thrown.
Sgt Tony Klenczar said: "This type of behaviour is dangerous and completely unacceptable.
"It is a miracle no-one has been hurt.
"If you know who did this you should report them to police immediately.
"We are working with the bus company and are carrying out operations and surveillance to prevent further incidents from taking place."