"Reckless" vandals have been throwing bricks at moving buses, smashing windows and causing thousands of pounds worth of damage.
Gangs have been targeting the vehicles leaving the Old Swan depot in Liverpool for 10 days, Arriva Merseyside said.
It described the attacks as "extreme acts of anti-social behaviour".
Vehicles were hit on Green Lane in the city, Pilch Lane in Dovecot, Middleway in Croxteth and on West Derby Road in Tuebrook.
Howard Farrall, Arriva Merseyside's area manager, said the company had allocated double-decker buses where possible to allow for social distancing, but the damaged vehicles were off-road for repairs, meaning fewer buses available for key workers.
"Over the past 10 days, Arriva has experienced several extreme acts of anti-social behaviour. These incidents have taken place across Merseyside and have consisted of young adults deliberately throwing items at moving buses.
"For anybody to recklessly endanger the safety of our drivers and customers is not acceptable."
Mr Farrall added Arriva is working with Merseyside Police. The force has been contacted for comment.