Police are hunting a man who can be seen throwing a punch at a fellow Tube passenger in shocking video footage.
Passengers can be heard screaming as the man launched his attack on a packed Northern Line train between Kennington and Stockwell at 10pm on Thursday night.
The woman who filmed what happened was on her way home from the theatre when the trouble started. She said the attacker pushed another passenger while getting on board the train, and the passenger responded by saying: 'Excuse me don't push'.
The witness said: 'I was so upset but I did something I've never done before and tried to film it because it was so violent and scary and I didn't want him to get away with it.
'The next minute the big guy turned round, started the verbal abuse again and then just launched a huge punch on the guy and started physically attacking him.'
Officers from British Transport Police are interviewing witnesses and examining CCTV. No arrests have yet been made.