A gang of four used a Samurai sword to hack a 20-year-old to death less than a mile from a second fatal knife attack in north-west London last night.
The victim, named locally as Sadiq Aadam, was found with serious stab wounds in Malden Road, in Belsize Park, around 10.15pm.
Witnesses said the gang fled on foot as passers-by desperately tried to save the stricken victim.
Just two hours earlier a 17-year-old was stabbed to death in front of his mother in the worst night of violence in the capital since New Year's Eve when four men died.
The teenager died in the street after being attacked outside Savers Mini Market corner shop in Bartholomew Road, Camden, at about 8.30pm.
Detectives from Scotland Yard's murder squad are investigating whether there is any link between the deaths. No arrests have been made.
There will be an increased police presence in the area to reduce the possibility of any 'retaliatory' attacks.
Both victims are believed to be children of Camden's Somalian community.
Witnesses reported seeing the suspects, with their faces covered by bandanas, arriving at the attacks in a van.
Police are awaiting formal identification of the first victim and said next of kin have been informed.
One resident, who didn't want to be named, told the London Evening Standard: 'The boy was playing football on the green just after 7pm. I see him playing football every day and going to school, and he went to the shop to get a drink.
'I heard there was blue van and someone jumped on him, and he got stabbed.
'His mum saw him die. What do you say to someone who just watched their child die? He's 17 - he's just a baby. He was never rude, he was lovely. I saw him taking his brother to play football.'
The woman added: 'He just went to the corner shop to get a drink and now he's just lying there. They put pictures of his dead body on the internet and there's a scorecard going around.'
Officers were later called to reports of a disturbance in Malden Road at about 10.15pm and found a man, believed to be about 20 years old, with serious stab wounds.
He was also pronounced dead at the scene. He was the 15th person stabbed to death in London since New Year's Eve. Six of them have been teenagers.
Describing the scene, a resident said: 'I heard a scream, a loud scream and I went to my window and he had been slashed with a samurai sword.
'There were four guys in a car and one jumped out and slashed him.
'People were screaming for towels, to throw down more and more towels, I threw out four.
'The blood was gushing out and people were desperately trying to stop it.
'The ambulance didn't come for about 30 minutes and the police were trying to resuscitate him, just pumping, pumping, pumping.'
One resident, who saw the attackers fleeing the scene, said: 'I saw these four kids in tracksuits running off. One turned to look at me, but they just kept running.
'I thought at first they were just playing around and then I heard the screaming.'
Another local resident added: 'I heard screaming and shouting and then I looked out my window and I could see him on the floor. There was blood everywhere and he was holding his abdomen to try and stop the bleeding.'
Denise Arnold, 63, who lives in Malden Road, said: 'My neighbour rushed out because she knew there was something bad. She stood on her bed and she could see.
'So she rang the 999 call and she was told to go down and check he was breathing.'
She said she saw a large group of people gathered around the victim, adding: 'There was a lot of screaming.'
At around 11.30pm she said she heard more 'wailing and screaming' at the scene.
Inquiries are under way to confirm the identity of the second man.
Witnesses reported seeing the suspects, with their faces covered by bandanas, arriving at the attacks in a van.
Detectives from the Homicide and Major Crime Command have launched separate investigations and said police are working to establish whether there are any links between the two incidents.
No arrests have been made.
A section 60 order - which gives police the right to search people when they believe serious violence will take place - was put in place overnight.
Inspector Paul Clarke said: 'There will be an increased police presence and an increase in the activity of the gangs unit to reduce the possibility that any one may consider retaliatory action.
'If anyone in the community knows anything about the incidents or the possibility of any ongoing occurrences they should contact police and let us know so that we can help to save lives before this kind of tragedy is repeated.'
The first attack took place just metres away from where a candlelit vigil was held for another teenage stabbing victim on Monday night.
Lewis Blackman, 19, who grew up on the Peckwater estate in Kentish Town, was killed in Kensington on Sunday outside Freddie Mercury's former home.
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