A man was knocked out amid an argument over face masks at a London railway station.
Police are investigating after William, not his real name, was punched by a fellow passenger at Clapham Junction station and required hospital treatment for a severe laceration and stitches for a head wound.
Along with a friend, William had been arguing with South Western Railway staff by the ticket barrier because one member was not wearing a mask.
Footage taken by the friend shows a white male in a grey hoodie pushing a pram past the heated exchange, he had a child with him and was also not wearing a mask.
William is heard saying "you're not wearing one either", which he told the PA news agency was not directed at the man.
In response, the man turns around and walks away from his pram shouting "don't f****** start on me" and punches William, knocking him to the floor unconscious.
The man quickly leaves the area with his pram and child, while William is seen bleeding on the ground.
A member of rail staff is heard saying: "I told you both to go didn't I?"
William told PA: "My friend asked one of the station staff a totally reasonable and polite question 'are you exempt from wearing a mask'.
"She'd snapped back saying 'none of your business'.
"Admittedly I lost my temper a bit.
"It escalated into a row with other station staff... then you can see the rest... thug leaves the scene, staff do nothing.
"No idea who he is, just some random person who was walking without a face covering assumed when I was talking to the guard just flew off straight away bang."
In the footage, staff did not pursue the man but could be heard saying they have called emergency services.
South Western Railway said the member of staff who was not wearing a mask had a medical exemption.
It said the assault was an altercation between members of the public and staff had assisted with first aid and alerted the police.
A spokesperson for South Western Railway, said: "Our staff reported this incident to the British Transport Police who are now investigating this.
"We will cooperate in this investigation fully, but as it is ongoing it would be inappropriate for us to comment further."
PA has contacted the British Transport Police for comment.