Police at the scene of the incident
Three youths, two aged 16 and one aged 17, have been arrested on suspicion of the murder of a 15-year-old boy who was stabbed in the chest in east London.
Stephen Lewis was found by police on Whitwell Road in Plaistow on Saturday night following a party at a church hall attended by about 100 people.
A group of youths ran off as the officers arrived.
The Metropolitan Police said Stephen was the first teenager to have met a violent death in London in 2009.
Police cordoned off the area between the junction of Barking Road and Balaam Street up to the Foresters Arms pub in Abbey Street.
Det Ch Insp Simon Moring said officers found the injured victim at about 2245 GMT, within minutes of a party ending, which he said was attended by up to 100 people at St Cedd's church hall.
He said: "I'm open minded as to what the motive might be.
Clearly this is a tragic stabbing of a young man who was in the prime of his life, who was just 15 years old. Absolutely tragic."
An aunt of the victim, who said her name was Michelle, gave a brief statement on behalf of Stephen's mother.
"Everybody loved him," she said. "They are going to miss him."
A local man, who did not want to be identified, said his nephew was a friend of the victim, and that both of them were "threatened".
He said: "He [the victim] was not a knife carrier. He just went to a party to meet his mates. But his mates weren't there and a group of lads came and he got stabbed."
A 15-year-old girl said the stabbing followed a row at a fund-raising event at the church hall.
Screaming and shouting
The party had to end when a fight broke out, she said.
"I saw pushing and arms swinging around. Then the bouncers came in and said the party has been knocked off.
"I was expecting a fight but I wasn't expecting anyone to get stabbed. There is always a fight, that is why I left early."
Another local resident, who saw the incident from her home, said: "There were between 50 to 100 kids and there was a lot of screaming and shouting."
Last year, 28 teenagers were killed in violent incidents in London.