A teenage boy and a man were stabbed to death last night amid a gathering at which people were reportedly taking laughing gas and filming a music video.
Residents in Islington, north London described the "intimidating" atmosphere as the group of around 20 people met at a car park, recorded the video and then became noisy and boisterous.
Mark, 39, said he saw water bottles being hurled at walls when the level of aggression "starting to up a little bit" by around 11pm.
And at 11.30pm, emergency services were called to reports of three people being stabbed. These include a 15-year-old boy who died at the scene and a man aged 23, who was declared dead at a hospital in west London.
The third victim - a 28-year-old man - remains in hospital with stab wounds.
"We don't get gatherings like this around here, it's normally quiet out," Mark told My London.
"There were 20 of them doing laughing gas. They were surprisingly subdued for a crowd that size. But my radar went off.
"There were water bottles getting thrown at the wall, they were starting to get boisterous. I could feel the aggression starting to up a little bit."
Photographs taken today at the scene show police officers investigating. Blood stains are visible in two places on a white Mercedes in the cordon.
An anonymous resident explained the gathering in the car park had initially been a group of friends filming a music video.
They said: "There was stuff going on outside for ages. They were filming a music video. And that was from 9pm onwards, all very friendly, a few neighbours went down and were chatting.
"There were lots of really good cars and stuff, it was all fine. And then later on there was lots of people doing laughing gas.
"Basically a few hours later around 11pm there was a lot of noise. We heard shouting that somebody had been stabbed. We did not see what happened.
"Initially it was a group of mates filming a music video and it was fine but then other people arrived. There was a guy who was holding a gun but we decided it was a water gun.
"It was intimidating even though it was a music video, there was a lot of people and a lot of shouting and the music was really loud. It's mad that people died."
Another anonymous resident told MyLondon what they saw last night, explaining: "You had around four to five expensive cars, mainly Mercedes, people dealing with each other and noise.
"There was an unusual number of gangs."
Further pictures from the scene show a bicycle discarded on the ground, with a grey Range Rover parked nearby.
No arrests have been made in connection with the deaths.
Local independent MP for Islington Jeremy Corbyn visited the scene today, where he said: "We have to deal with the issue of knife crime and violence in our society.
"We have to have a police force that is ready to help people but we also need that community buy-in where people simply say to anyone, carrying a knife will not defend you, not protect you, will not make your life better.
"It will make your life more dangerous and you more vulnerable."
Chief Superintendent Andy Carter, of Metropolitan Police, said: "My thoughts are with the families of the two victims who have suffered such an unimaginable loss in this shocking and unacceptable incident.
"We will support our colleagues as they investigate and members of the public will see an increased policing presence in the area.
"I urge anyone who has information that can help us establish exactly what has happened to approach those officers or contact us immediately.
"A man and boy have lost their lives and we must now work together to identify those responsible and remove them from our streets.
"Anyone who has information that could help police should call 101 or tweet @MetCC ref CAD 9383/29 Jun. To remain 100% anonymous please call the independent charity Crimestoppers on 0800 555 111."
