A woman broke down in court today as she recalled the Halloween night horror during which her boyfriend was killed.
Allanna Devine, 28, wept in the witness box as she said she ran back to see Ben Gardner had been attacked by a gang.
Miss Devine said she had gone down the road to confront a man who had insulted her after her Halloween hat and wig were stolen - then heard a friend scream.
She said: "I heard Kay screaming. I ran back round to see Ben."
After being given tissues and water behind the screen where she was giving evidence at the Old Bailey, Miss Devine said she could not recollect everything that happened.
Mr Gardner, a 30-year-old computer expert, died in hospital from brain damage after being kicked and punched.
Judge Richard Hone told the jury that what had caused the fatal brain damage would be subject to expert evidence.
Daniel Ransom, 21, Ross Collender, 21, and Jordan Dixon, 18, all of Carshalton, south London, deny murder.
Mr Gardner, who worked for insurance firm Legal & General, had been to a club with Miss Devine and friends to celebrate her birthday on October 31 last year.
Earlier, Miss Devine said she had known Mr Gardner for three months and had been his girlfriend for a few weeks.
She said: "I was in a pirate's sort of fancy dress outfit with a black wig and black pirate's hat."
Her friend Kay Chick had a red wig and Mr Gardner had a leather jacket and vampire's fangs.
After going to a nightclub, they were going to her home in Sutton, Surrey, when Mr Gardner and Miss Chick went into a kebab shop while she sat on the steps waiting for them.
She added: "I was texting a friend. I was aware of the group coming towards me.
"My hat and my wig were taken from my head. I was not concerned about it because it was the end of the evening, but I did say 'Can I have my hat back, please?'
"I was greeted by jeering. They were very loud."
After getting near to her flat, her companions asked what had happened to her wig and hat, the court heard.
She told them: "A group have just taken them. We were walking towards my flat.
"Ben decided he was not happy that my wig and hat had been stolen and he was going to approach the group about it."
As he went off, the verbal abuse increased and she heard herself being called "a big fat moose", before she saw her boyfriend returning, she told the court.
She said he had her wig. Earlier, she had seen one of the youths wearing it, the court heard.
Miss Devine said she had decided to approach a youth who was calling her names and asked him: "What is your problem?"
She continued: "I was hit in the face by him. I was fuming. I said 'You are a big man to hit a woman'."
She told the court she was aware of Mr Gardner behind her and the group "flared up".
Miss Devine said she remembered one of the men jumping up and down and taking his jacket off.
She told the court Mr Gardner had said: "I am not a fighter but I have just seen someone hit my girlfriend and I am not standing back and letting you hit my girlfriend."
Brian Altman, QC, prosecuting, has told the court that CCTV footage showed the incident unfold.
He said: "As Miss Devine went to pick up a black witch's hat which had been thrown by someone unconnected into the road, Dixon booted it away.
"Ransom kicked and stamped on it, and in a quite provocative and contemptuous act, spat in it and threw it at her."
Then Collender "suddenly and unexpectedly" punched Mr Gardner on the left side of the face, Mr Altman said.
As he recoiled from the blow, Mr Gardner was punched by Dixon and fell on to his back in the road, it was alleged.
Mr Altman said Ransom then kicked an "utterly defenceless" Mr Gardner - described by witnesses as being like someone taking a penalty kick to his head.