A bus driver and ticket inspector were kept in hospital overnight after being attacked on a Glider in west Belfast on Thursday night.
The ticket inspector was assaulted and dragged off the bus by a group of six to eight youths who had become abusive.
When the driver got out to help him a larger group of youths joined in the attack. The driver was hit with a brick during the assault.
The attack happened at Northumberland Street at about 20:50 GMT.
Davy Thompson of the Unite union said the attack on the two staff members was a disgrace.
"They're hurt, they're sore, they're bruised. They're both at home now, both were released [from hospital] early this morning," he said
"There doesn't seem to have been anything that kicked it off, there's no money taken, it just seems to be abusive anti-social behaviour by these young people.
"It's been a regular occurrence over the last year, 18 months.
"Services into Belfast were actually stopped for a period of time and community workers got involved with Translink trying to resolve that and we would call on them at this time to actually get involved and try and identify these people that caused this last night."
He said the attack would have been a traumatic experience for the victims.
"We know from past history that people that have been involved in these types of assaults haven't gone back to their duties, they've ended up doing something else within the company," he said.
"Is this the type of society we want to live in, where you have elderly people, young couples with young children, scared to get on the buses?"
Translink said it "strongly condemns" the incident.
"The safety and welfare of our staff and passengers is our top priority and we are assisting the PSNI with their investigation," the company said.
"We operate a cash reward scheme offering up to £1,000 to any member of the public who witnesses any anti-social incident against our staff, vehicles or property and is prepared to give evidence in court, resulting in a conviction."
Police have appealed for anyone who witnessed the attack to contact them.