Tulisa Contostavlos has been in court accused of swinging her arms towards a man at a music festival, hitting his left eye.
Celebrity blogger Savvas Morgan said he suffered swelling to the eye after the incident at the V Festival in Chelmsford, Essex, last year.
Contostavlos has denied an assault charge linked to the alleged altercation last August. The 25-year-old appeared at Chelmsford Magistrates' Court with her assistant Gareth Varey, who is accused of threatening behaviour, which he also denies.
Mr Morgan told District Judge Angus Hamilton that Contostavlos raised her fists in the air and was swinging her arms.
"Everyone was jumping around trying to get at me, there was a lot going on," he said.
Other people's arms were also swinging around, but they were not as close as those of Contostavlos.
Asked if one of her limbs made contact with his eye, he said: "Yes."
He said he was led away by security: "Yes, he said 'There's one of you and 15 of them, and we don't want to see you get hurt'."
Prosecutor Matthew McNiff said the incident happened at 12.30am on August 18 as Mr Morgan was about to leave. He said Contostavlos accused Mr Morgan of stalking her, and swore at him.
"She saw the opportunity to hit him, and hit him in the eye," he said.
Contostavlos's defence was that she had been careful to keep her arms behind her back, and someone else assaulted Mr Morgan, he added.
Mr Morgan said he had never been accused of stalking anyone else in the business.
Security officer Aaron Smythe said he was in a mobile response team in a minibus at the festival: "There was quite a large-scale incident, and we went over to assist."
He said there was a large black man who was near a woman.
"The lady clearly was the aggressor, she ran to him and caught him on the head with the side of her hand," he said, before identifying Contostavlos in court as the woman involved.
As for the man who was hit, he said: "He appeared shocked and quite upset. He was holding his face for a couple of seconds just after the incident occurred."
The trial was adjourned until tomorrow when the prosecution will close its case and it is expected Contostavlos will give evidence.
Contostavlos and Varey, 28, both of Royal Drive, Friern Barnet, north London, are both on unconditional bail.