Angus said the man standing on the platform appeared to be "leaning forward with his arms out, like in a zombie position" while he was being sprayed in the eyes.

"He [the officer] sprayed the dude two to three times right in the eyes and I could see even through the train window ... the area around his eyes and nose was soaked in pepper spray and then after that he started stumbling around disoriented.

"Then one of the other officers extended his baton and hit the man once or twice behind the knee caps ... to bring him down while the other officer who had been pepper spraying him started restraining him.

"I started filming and the PSO with the baton kept on going to down on his legs and his shoulders and torso with the baton.

"It was pretty distressing to be honest, it was pretty shocking to see ... [some] of the bystanders was a child and a mother who were standing right adjacent to the beating."

But Victoria Police Commissioner Graham Ashton said the PSOs approached the man after he threatened the officers, including threatening to throw them off the platform and onto the tracks.

Mr Ashton said the man assaulted one of the officers, who was left with cuts and bruises to his nose.

"These matters are referred to a professional standards command to look at and they weigh it up," Mr Ashton told the ABC.

A police spokesman said the patron was a 26-year-old man with no fixed address who resisted arrest.

He said the capsicum spray and batons were used to subdue him. He has been charged with assaulting an emergency services worker and bailed to face court at a later date.

Northcote MP Lidia Thorpe said the footage was "deeply disturbing".

"We all know Merri station to be a quiet and serene place; it’s shocking that such an act of violence happened here. There is no excuse for the use of unreasonable force," she wrote on Twitter.

PSOs have been under scrutiny for their use of force, with an Independent Broad-based Anti-corruption Commission investigation in 2016 finding that assaults and excessive force, the unauthorised pursuit of information and predatory behaviour were key concerns.

In 2015, Victoria Police launched an investigation into the actions of two PSOs after footage emerged showing officers repeatedly spraying two teenage boys with capsicum foam because one did not have a valid train ticket.

That same year, two PSOs allegedly assaulted a man after he refused to let them search his bag.

PSOs have themselves been assaulted, with an anonymous 53-year-old officer telling media in late January that he was punched in the face after telling a patron to leave Carrum station over drunk behaviour.

Late last year, police charged three people after a PSO was allegedly bitten in an early morning attack outside Flinders Street Station.

Anthony Kelly, the executive officer at Flemington Legal Centre that runs a program specialising in police accountability, said he was concerned about the possibility of the PSO carrying out overhead baton strikes.

"It looks like from the footage that some of the initial strikes are towards the head and shoulders.

"Baton hits to any vulnerable areas, particularly over the shoulders, are of great concern."

Mr Kelly said his firm had helped a series of clients over the years who have reported excessive force from PSOs.

"We continue to find that low level ticketing offences are the primary tasks of the PSOs ... their presence makes it more likely that there will be incidents occurring that can escalate like this," he said.

"The PSOs are armed so we’re conscious there could be a critical incident at some point."

Since 2011, the armed PSOs have been employed by Victoria Police to patrol train stations to maintain public safety.

They have the authority to arrest and detain people, request personal information, and search people and their property.

PSOs have the power to use reasonable force to carry out their duties, according to police behaviour guidelines.

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