A number of bus services were cancelled after several incidents of drivers and moving buses being attacked on Halloween.
The company said that at least two incidents involved "large objects being thrown directly at the driver" but added that no injuries had been reported.
Some services were cancelled amid fears more attacks could take place.
Thousands of pounds of repairs now have to be carried out on a number of buses, according to the company.
The company said the Llanelli to Swansea service was attacked in the Castle Street area of Loughor, near Swansea.
In a separate incident, the Port Tennant to Swansea service was reportedly disrupted by a person dressed in a Halloween costume blocking the route of a moving bus.
Rob Pymm, First Bus Wales and West's commercial director, described the behaviour of a minority as "despicable".
"The images we have seen of our buses from last night speak for themselves; who in their right mind would consider throwing a heavy object at a moving bus, let alone towards the driver, endangering them and everyone on board," he said.
"Sadly, we had no option but to pull some routes last night, and today our passengers continue to be inconvenienced while the buses are off the road being repaired.
"All we can do is apologise and reassure the public that we will resume a normal service as soon as possible."
First Bus Wales and West said it had reported the incidents to the police and was reviewing its on-board CCTV footage.