A day of violence saw two teenagers killed within minutes of each other in different parts of London while a man was stabbed to death hours later.
Three men were also stabbed in a separate attack during a weekend of violence in the capital.
Scotland Yard said a man in his 30s died after he was stabbed in east London on Saturday afternoon.
Police officers and London Ambulance Service attended reports of an injured man at Alton Street, Tower Hamlets, just before 2pm.
He was pronounced dead at the scene – a field next to a children’s nursery and a mosque – some 40 minutes later and police began inquiries to notify his next of kin.
A crime scene has been established, there have been no arrests and officers retain an open mind as to the motive for the stabbing, the Met said.
The force said 14 people – including several boys and a girl – have been arrested in connection with three other incidents, two of which happened on Friday afternoon and the other in the early hours of Saturday morning.
London Mayor Sadiq Khan said he was “sickened” following the deaths of the two teenage boys.
Emergency services were called to Deeside Road in Wandsworth, south-west London, at 4.42pm on Friday where the first victim – aged 18 – was found stabbed.
The victim died at the scene and is yet to be formally identified, with a post-mortem examination due to be held.
Six males – aged between 16 and 19 – were later arrested and taken to separate west London police stations for questioning.
Marina Paget, 22, who lives in the area, told the Press Association: “I walked past at 2.30pm and I didn’t see anything, but when I came back from work at 5.30pm it was all going on.
“I walked past I saw what looked like the body on the floor covered in a bag and there were 50 people around, loads of people, two or three ambulances.”
In a second incident, at 4.54pm, armed police were called to Hartville Road in Plumstead, to reports of a shooting.
The victim, aged 19, was found with critical injuries.
The London Ambulance and the London Air Ambulance attended, but the teenager died at the scene a short while later.
The victim has yet to be formally identified and police were working to notify his next of kin.
Three boys and a girl, all aged between 16 and 17, were later arrested on suspicion of murder and taken to separate south London police stations for questioning.
At around 3.22am on Saturday, police were called to Bedford Road in Clapham following reports of a fight.
Three men were found with stab and slash injuries. The condition of two of the men is unknown, while the third has injuries which are not life-threatening or life-changing.
Four men were arrested in connection with the incident – two for violent disorder, one accused of carrying a bladed instrument and another for possession of a Taser – and have been taken into custody at a south London police station.
Mr Khan said: “I am sickened to hear that two young lives have been ended within minutes of each other in Wandsworth and Greenwich.
“Our overstretched police are working around the clock to keep Londoners safe. They need our support to end this scourge of violence.
“Anybody who has information about what happened either in Wandsworth or Greenwich should do the right thing and call the police.”
In a statement, the Met said “bearing down on violent crime” in the capital remains “their top priority” but appealed to the public for information about people carrying knives.
The force added: “Teams across the city are working tirelessly – day and night – to identify and pursue offenders, help bring perpetrators to justice, take weapons off the street, support victims, engage and reassure the public, and keep our communities safe.
“However, the police cannot solve this problem alone. The police, our partners and the public must work together to deal with knife crime and prevent further lives being lost.
“Communities have a vital role to play in tackling violent crime. We want to hear from anyone who may have information about people who may carry knives or firearms, or who are involved in organised drug crime and violence.”