Two teenage boys have been killed in separate stabbings amid concerns about a surge in violence over the summer.
A 15-year-old died in Woolwich, south-east London, and a 16-year-old in Lambeth, south London, on Monday.
Mayor of London Sadiq Khan said he was “heartbroken and appalled” by the deaths.
Police have been gearing up for an increase in violence during the summer months after a surge last year when lockdown measures were eased.
In April, officers staged a national crackdown on knife crime, Operation Sceptre, which included deploying knife arches and sniffer dogs as well as encouraging the public to hand in weapons.
At the time, Metropolitan Police Commander Alex Murray told the PA news agency: “We fully expect, as people come back on to the street, and with summer approaching, that people might be tempted to engage in violence, or the ingredients might be there.
“We’ve been spending a lot of the lockdown period preparing and getting ready to suppress as much violence as possible.”
The force said the 15-year-old boy died after being stabbed in Woolwich New Road in Woolwich, south-east London, shortly after 5.20pm.
Violence erupted in a busy area near Woolwich Arsenal station and pictures on social media showed members of the public holding up sheets to provide privacy as emergency crews tried to save the boy’s life.
Another 15-year-old, who later took himself to hospital suffering from a knife wound, has been arrested on suspicion of murder.
In Lambeth, a 16-year-old boy was pronounced dead at the scene after being found injured in Oval Place at around 11.45pm on Monday.
Police officers were at two separate cordons near the scene on Tuesday.
A tarpaulin sheet covered a stairwell at Elworth House, a block of flats just off the main Clapham Road.
Members of the public arrived looking visibly distraught and placed flowers with personal messages.
Cards read “shine bright” and “love you forever … my bro.”
A resident of Elworth House described the incident as “shocking” but “not uncommon”.
“You always have issues here… it’s really bad,” he told the PA news agency.
“Unfortunately, as shocking as it is it’s not uncommon. It’s really sad.”
Another resident, aged 25, said: “I watched that little boy grow up. He was a good boy… he had his own mindset, you know. Bless him.”
Superintendent Petra Lazar said of the Woolwich incident: “It is shocking and extremely saddening that a teenage boy has lost his life as the result of knife crime.
“My thoughts at this time are with the family of the boy who has died.
“This incident will understandably cause worry and concern to local residents and I would like to reassure them my officers are supporting the murder detectives in every way we can.
“Increased patrols will be carried out in the area over the next few days.”
The Mayor said he is in “close contact” with Metropolitan Police Commissioner Dame Cressida Dick as her officers investigate the stabbings.
He said: “I am heartbroken and appalled by the deaths of two teenagers in our city. Every loss of life leaves a family and a community devastated.
“My thoughts remain with the victims’ family and friends at this extremely difficult time.
“I am determined to do everything in my power to support the police and our communities to put an end to these horrendous crimes and keep our young people safe. This violence has no place in our city. It must stop.
“I am in close contact with the Met Commissioner, and the police are doing everything possible to bring those responsible to justice.”