Graphic footage has emerged of two protective services officers (PSOs) allegedly assaulting a man at a suburban railway station in Melbourne's north during an arrest.
The video, which was filmed by a passenger on a train, shows people watching as the two officers grapple with the man on the platform of Merri station at Northcote.
A witness told ABC Radio Melbourne one PSO swung a baton at the man's legs.
The sound of the baton hitting him can be heard on the video footage, which was taken from inside the carriage through the open doors of the train.
"It was very, very shocking and like, quite distressing to see," Angus, who filmed the alleged assault, said.
Angus said he was heading home on the train after dinner on Wednesday at 8:00pm, when he saw the man talking to PSOs at the station on the South Morang line.
He said the officers pepper sprayed the man.
"After seeing him be sprayed twice in the face, I could see a second PSO pull out his baton and start hitting the man in the kneecaps when he was disoriented," he said.
"He didn't seem to be attacking the PSOs."
PSOs are trained at the police academy. They patrol public spaces such as train stations to boost security and community safety.
But Victorian Police Commissioner Graham Ashton backed the PSOs involved in the incident, saying the man had threatened to throw the officers off the platform.
"Then this particular fellow, as I'm briefed, we allege that he's assaulted the PSO," Mr Ashton told ABC Radio Melbourne.
"The PSO's got cuts and bruises to his nose from that assault and they've arrested that individual."
Commissioner Ashton said the graphic footage showed the man's arrest.
"When people are arrested and they resist arrest, they don't just put their hands behind their back to be arrested from an assault like that. It can be very confronting," he said.
"These sort of matters, they are referred, if we get footage like that, we always send them into the Professional Standards Command to have a look at, and they weigh that up in terms of what's happened at the events."
'Terrible footage' being investigated: Police Minister
Police Minister Lisa Neville described the footage as concerning and said the matter would be "fully investigated".
"These are matters that are taken very, very seriously. Of course we've seen one part of the footage and it's terrible.
"If this is inappropriate force then, certainly I know the Chief Commissioner takes that very seriously and will take action."
The Greens MP for Northcote Lidia Thorpe said the footage was "deeply disturbing".
"The Victorian Greens have often raised concerns about PSOs on railway stations and the disparity between the training they receive and the powers they hold," she said.
"They're often dealing with vulnerable members of the community and their role should be to de-escalate the situation without using unreasonable force."
She said even though Commissioner Ashton said it happened after the man had resisted arrest, it was "hard to think of a situation where this reaction is warranted".
The Greens have called for the anti-corruption watchdog to investigate.