A schoolboy was knifed to death at a birthday party after hundreds of teens were invited on Snapchat, a court heard.
Jordan Douherty, 15, was murdered during a fight outside a community centre after the crowds were kicked out, jurors were told.
A trial at the Old Bailey heard he was chased and beaten before a teenager stabbed him twice while he lay “defenceless” on the ground.
A 16-year-old boy, who cannot be named for legal reasons, accused of stabbing Mr Douherty denies murder.
Two other boys, 16 and 17, are also on trial accused of conspiracy to commit GBH.
Jurors heard the boys all went to a girl’s sixteenth birthday party at North Romford Community Centre in east London on June 23 this year.
Duncan Atkinson, prosecuting, said: “The invitation to her party had been posted publicly on Snapchat, with the result that a very large number of people attended.
“Indeed, the number who attended was far too large for the community centre, so that at 9pm the centre was closed.”
As the crowd spilled out into the street, Jordan was chased after a confrontation with the three defendants, the jury heard.
Mr Atkinson said: “These events unfolded suddenly, unexpectedly and at considerable speed.”
He said witnesses had “to make sense of fast and shocking events unfolding without warning right in front of them.”
“These events were also witnessed by a number of police officers who had attended to disperse the large crowd that had attended the community centre,” Mr Atkinson said.
CCTV and police body-worn cameras also filmed the chaos, the jury heard.
Mr Atkinson added: “From this large group a smaller group of young males then surrounded Jordan at a time when he fell to the ground.
“The members of that smaller group proceeded to inflict punches and stamps on him.
“Again, the evidence, including both CCTV from the area and a video recorded on a mobile telephone that was distributed on Snapchat, shows the defendants to be a part of this smaller group that surrounded and then attacked Jordan Douherty.”
The 17-year-old defendant “could be seen to be delivering kicks to Jordan as he lay on the ground” and stamping on him, the court heard.
The 16-year-old accused of GBH “ran up at the time Jordan went to the ground and also started kicking towards him, again with repeated stamping movements.”
Mr Atkinson said: “They had joined with the others in a joint attack in which, using their shod feet as weapons on a defenceless 15-year-old lying on the ground, they were together seeking to inflict, as an inevitable consequence of their joint actions, serious harm on him.”
The boy accused of murder then “took matters significantly further because he was armed with a knife,” the jury heard.
Mr Atkinson said: “During the course of this group attack on the defenceless and prone Jordan Douherty he was stabbed.
“At that time Jordan posed no threat to anyone and the use of a knife was wholly even the beginning of a justification.
“Jordan received two stab wounds to the chest area.
“One entered the left ventricle of the heart and that catastrophic injury proved quickly fatal.
“The other entered the chest in an upward trajectory, cutting a vein in the neck area.
“This on its own may have been fatal but for that other injury that made Jordan’s death inevitable.”
CCTV, the Snapchat footage and eyewitnesses all showed it was the accused holding the knife, the jury heard.
Mr Atkinson said: “That significant escalation in the violence, from the taking of steps to cause serious harm to taking of steps that caused death, arose all too suddenly in the context of spontaneous group violence.
“It was the action only of the person who wielded the knife and used it to kill.
“Immediately realising the enormity of what he had just done and no doubt conscious of the presence at the scene of a growing number of police officers, [he] sought to get rid of the knife he had just used to inflict the fatal injury.”
The blade was handed to another person who has since been “dealt with in relation to his handling of the knife”, the court heard.
Mr Atkinson said each of the defendants later told they were at the party but deny involvement.
The trial continues.
By Lewis Pennock