Vandals have targeted six buses in a two-day spree of "mindless violence".
Gwent Police said stones were hurled at vehicles in Newport, in some cases smashing windows.
Newport Bus and the NAT Group said their buses were bombarded over Monday and Tuesday, including twice in broad daylight.
Police have now been deployed to travel on the services. Bus routes were changed because of the attacks but are now back to normal.
Newport Bus managing director Scott Pearson said: "On Monday night, due to two buses damaged by rocks thrown at the windows in the Ringland area, the bus route was modified to protect our staff and passengers.
"What is more concerning is the attack that took place on Tuesday afternoon.
"This time, projectiles were thrown at the front of two separate buses."
Mr Pearson said it was "extremely lucky" the windscreen was not broken.
"We cannot stress enough how dangerous this type of behaviour is to all involved," he added.
"One of the perpetrators of Tuesday's attack has been recognised and their details have been passed to the police who have also advised that from Wednesday PCSOs and officers will be on duty on the buses."
NAT Group managing director, Adam Keen, condemned the vandalism as "unacceptable".
He said he did not "want to alarm" passengers but wanted to make them aware of the situation.
"There does seem to be a certain demographic responsible for the recent attacks, and I think the combination of Halloween and half term has exacerbated the issue," he said.
Gwent Police Sgt Chris Back said the attacks were "unacceptable."
"We will not tolerate the safety of others being put in jeopardy in this way," he said.
He said it was "extremely fortunate" there were no injuries.