A teenager “collapsed as though he had been hit by a train” after being stabbed in the heart by a boy in school uniform on a packed bus, a court heard this week.

Jurors were shown CCTV footage of the attack, and the trial was halted for 10 minutes after one of them burst into tears.

Derek Boateng was knifed in front of terrified passengers on the No 393 bus in Highbury New Park, in April, and died the following day – on his 16th birthday.

The 16-year-old boy accused of his murder, who cannot be named for legal reasons, is said to have boarded the bus and made a beeline for Derek, stabbing him with a kitchen knife six to eight inches long.

He admits knifing the teenager, who also had a blade, in the chest but denies murder – claiming he was acting in self-defence.

Screams

Sian Harrington, a fellow passenger on the bus, said: “It was kicking off behind me, screaming and shouting.

“I looked up and there was a boy just slightly in front of me with a knife in his hand lunging forward.”

The driver stopped the bus and the knifeman is said to have left the vehicle, mouthing expletives at his victim through the window. His victim remained standing for “a minute or so” after being stabbed.

Ms Harrington continued: “Then it was as though he had been hit by a train. He collapsed and fell back in his seat with his head against the window

“Myself and another woman decided we would try and lay him down. We didn’t know what had gone on and I thought he was in shock or fitting.

She added: “His eyes were open and I asked him his name. The other woman was putting pressure on the wound while I held his head and talked to him.”

Earlier prosecutor Tom Kark said witnesses saw the two boys slashing at each other.

He said: “The single wound which killed him was caused by this defendant, and the blade penetrated the right lower part of his chest and pierced, unfortunately, the right ventricle of his heart.

“As a result, Derek Boateng suffered massive internal blood loss.”

Senseless

He said there appeared to have been an earlier dispute between the pair, but added: “Whatever had gone on before, we say this was a senseless act of violence.

“It resulted in the tragic and untimely death of a young boy.”

The defendant has admitted stabbing Derek but claims he was acting in self-defence.

David Howker QC, defending, suggested Ms Harrington had witnessed an argument between the two boys and only seen the knife after the confrontation.

She said: “My first recollection is that I looked up and saw a knife. I thought to myself, ‘it’s happening. I am on a bus in London and someone has a knife’.”

The trial continues.