A student accused of killing a gay man in Trafalgar Square said he punched the victim because he saw him strike a girl and feared she might get hurt.
Joel Alexander, 19, told an Old Bailey jury he felt he "had to act" after he saw Ruby Thomas in a confrontation with 62-year-old Ian Baynham.
He said: "What I was perceiving was a grown man hitting a girl. I thought Ruby could end up getting hurt because he was quite substantially bigger than her."
Mr Baynham died 18 days after being knocked to the ground where he was allegedly kicked by Thomas and a second girl, Rachel Burke, in September last year.
Thomas, 18, of Lichfield, Staffs, Alexander, of Thornton Heath, south London, and Burke, 18, of Three Oaks, East Sussex, all deny manslaughter.
Alexander, a sports science student at the University of East London, today told the court how he had been out with his girlfriend Louisa Figini and others celebrating her birthday and drinking.
He said before the altercation with Mr Baynham there had been another row in Trafalgar Square involving "lairy, mouthy" Thomas and a group of youths which he had tried to defuse by saying: "They are just drunk, silly girls."
Nothing more came of it, Alexander said, adding: "I couldn't predict that there was going to be more trouble."
The first thing he knew of the later row "was when Ruby shouted something like 'batty boys"' at Mr Baynham and a friend, he added.
"I remember him turning around and saying something back to Ruby along the lines of 'I may be gay but...'.
"Then I remember him walking a few paces towards where we were sitting. They walked towards each other."
Alexander said there were raised voiced and that it seemed there was about to be another confrontation, and that then he saw the contents of Thomas's bag go to the floor.
"The next thing I saw was Mr Baynham in the same motion throw his arm and his foot at Ruby. I saw the foot connect with the shin but I didn't see the arm connect. It looked like it was aimed towards her upper body.
"I was already concerned, I was already moving towards the situation.
"What I was perceiving was a grown man hitting a girl. She was more subdued, she wasn't as lairy any more.
"It looked to me as if the man was starting to get the better of her. That's when I thought I had to act. I lunged at Mr Baynham and hit him with my right hand."
Alexander told jurors that it happened in a "split second" and it was "a very fast-moving situation".
His barrister Kerim Fuad QC asked: "Did you have any concerns about what would happen if you didn't act?"
Alexander replied: "I thought Ruby could end up getting hurt because he was quite substantially bigger than her."
He said he connected with Mr Baynham's jaw with "moderate" force and did not think it would knock him to the ground.
"I saw him go back to his right foot and then backwards," said the defendant.
Mr Fuad asked him why he got involved and why he did not turn a blind eye, and Alexander replied: "In my eyes, in my opinion, a grown man shouldn't hit a girl."