Mr Hopkins cried in court as he recalled how Mr Knox (right) died last May
A witness has told the Old Bailey how a "madman" repeatedly stabbed his friend, teenage Harry Potter actor Rob Knox, outside a pub.
Tom Hopkins was also stabbed in May's attack in Sidcup, south-east London.
He alleged Karl Bishop waved a knife and shouted in an aggressive manner which created a "feeling of danger" among the crowd outside the Metro bar.
Mr Bishop, of Sidcup, denies murdering Mr Knox, who was 18 when he was killed, saying he acted in self-defence.
Mr Bishop, who is 21 and lives in Carlton Road, also denies five charges of wounding with intent to cause grievous bodily harm to five other youths.
Jurors have been told that Mr Knox was knifed five times as he tried to protect his younger brother Jamie.
This was allegedly after hearing that Mr Bishop had threatened Jamie with a knife.
Having been restrained by friends, Mr Knox "broke free and advanced towards the man with the knife", Mr Hopkins said.
"He was in a running movement. There was a clash. Rob was trying to fight him, there was grappling."
Mr Hopkins, 19, told the court that Mr Bishop "looked like a madman".
"People in the crowd were shouting, 'He's got a knife, he's got a knife'," he said.
The hearing was halted for several minutes when Mr Hopkins began crying as he gave evidence, at the point when he was describing his friend's death.
He went on to say: "As Rob was grappling with him, the boy had a knife in the right hand. He was stabbing Rob repeatedly."
Another friend grabbed Mr Bishop and flung him into a flowerbed, pinning down one of his arms, the court was told.
Mr Hopkins said he suffered a stab wound himself - to his head - when he tried to grab the knife from Mr Bishop's hand.
"As I was in that movement I felt a blow to the right side of the back of my head. I just felt blood," he said.
"It wasn't just a little bit, it was a massive amount of blood."
Later the wound was treated in hospital, the court heard.
The trial continues.