An alarming video has emerged of a man repeatedly punching a young passenger on a Transperth bus.

Footage of the incident, which happened on the Rockingham 555 bus at 3.30pm on Friday, shows a bearded man laying into a fellow passenger - initially attacking him towards the front of the vehicle before pursuing him towards the back of it.

Alarmed and angry commuters can be heard yelling at the man to get off the bus.

Transperth has confirmed the incident happened on the 555 and that its security officers responded and escorted the bus until the man alighted at the corner of Council Avenue and Contest Avenue in Rockingham.

According to Yahoo 7 News, police were called to the scene. WA Police have been contacted for clarification.

A young woman who posted the video online said the man had focused on her and friends because of their Maori origins and that the passenger being punched was a 17-year-old.

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Public Transport Authority spokesman David Hynes said the safety of public transport users and staff was top priority and that it was committed to tackling anti-social behaviour.

“We have a number of systems in place to protect passengers and staff in the event of an incident on our buses,” Mr Hynes said.

“This includes drivers being able to radio through to the depot, who can contact police, or to our security contractors.

“As well as on board CCTV and two-way radios, all our buses have GPS-linked duress alarms which can be discreetly operated by the driver.”