Three police vans have been vandalised while officers were conducting a knife-sweep patrol.
The unoccupied vehicles were damaged in Heath Avenue, Salford at about 16:00 GMT on Friday.
Officers had discovered "a number of offensive items" before returning to find the vehicles' windscreens shattered and tyres damaged.
No-one was hurt, Greater Manchester Police said as officers appealed for witnesses to contact them.
Ch Insp Ben Ewart said: "This behaviour is completely unacceptable and it will absolutely not be tolerated.
"Despite this incident, you will continue to see patrols in and around the area as our officers carry out their positive work within the community."