Officers were alerted to an incident in High Street, Wealdstone, Harrow, at 1.17pm on Sunday and found a 15-year-old boy with a gunshot wound.
Two minutes later, nearby in the same street, a 13-year-old boy was also treated for gunshot wounds. He had suffered a shotgun pellet wound to the head.
The MP for Harrow West has pledged to work with police to find those responsible.
“Shocked & horrified by reports of two children shot in my constituency,” Gareth Thomas said on Twitter.
“My thoughts are with them & their families who must be worried sick. I will be working to support the police investigate what happened, why & catch those responsible.”
Both boys suffered head injuries and were taken to hospital. Police have since confirmed their injuries are not life-threatening.
Officers, who initially believed the 13-year-old was 12, are investigating whether the two incidents are linked.
In a statement, police said no arrests had been made and no weapons had been recovered.
People were asked to avoid the area while police investigated the incident.
At the scene, a grey Adidas sweatshirt with blood stains on the hood could be seen crumpled on the ground near a Specsavers branch, along with a police exhibit number.
One shopkeeper, who believed a bullet had grazed the back of the 13-year-old’s head, said the youngster was “lucky to be alive”.
The manager of a betting shop nearby spoke of one of the boys being “proper covered in blood” but said he was “sitting calmly as the paramedics were looking after him”.
Three-and-a-half hours later, emergency services were called to reports of a 20-year-old man suffering serious stab wounds after being attacked in Luton, Bedfordshire.
The victim was pronounced dead at the scene and officers have launched a murder investigation.
Detective Chief Inspector Phil Moss, senior investigating officer, said: “This is clearly a shocking incident where a man has been fatally wounded in a public place in broad daylight.
“The community will be understandably concerned and I’d like to reassure people we are taking this matter extremely seriously and have increased patrols in the area.”
In a separate attack at about 6.30pm, a 22-year-old man was wounded in a shooting in Lewisham, south London. His condition is not life-threatening.
And in a further unrelated attack at about 9pm, a 43-year-old man was stabbed in a residential street in Perivale, northwest London.
The stabbing is understood to have taken place after an altercation over his driving. His injuries are not believed to be life-threatening.
The attacks come a day after Rhyhiem Ainsworth Barton, 17, was fatally shot in an unconnected attack in Southwark, south London.
Armed police attended the scene and he was given first aid by paramedics but died shortly before 7pm.
Rhyhiem’s mother, Pretana Morgan, paid tribute to her son – an aspiring architect and rapper – and criticised police for “putting us in danger”.
Labour MP David Lammy tweeted: “Enough. Enough. My heart goes out to families grieving children and teenagers. So many shattered lives, families and communities.”
Meanwhile, three people were injured in a “noxious substance” assault following an altercation between two groups in east London at around 5.20am on Sunday.
Police are investigating whether the men, aged 22 and 27, and a 17-year-old boy, suffered life-changing injuries in the incident in Hackney.