The number of homicides in London so far in 2018 is the highest in a calendar year this decade.
In the latest violent death in the capital, an 18-year-old man was stabbed to death in a fight in Alwold Crescent, Lee, south-east London, shortly before 10pm on Tuesday.
Police said they were called to "reports of armed youths in the street". A second man, also 18, went to hospital with stab injuries but has since been discharged.
Three men aged 22, 31 and 56 and two women aged 29 and 53 were arrested and taken to police stations in south London for questioning.
Detective Chief Inspector Simon Harding said: "We are at the very early stages of our investigation into this tragic murder. We urgently need to hear from anyone who saw what happened in Alwold Crescent.
"We have already spoken to a number of people as part of our initial inquiries, but we would like to hear from anyone who lives near the scene who saw anything that might assist our investigation to contact us.
"Did you see either group going to and from the scene? Did you see the fight? Perhaps you saw something after the incident that in retrospect may be linked. We would urge you to get in touch right away."
125 violent deaths
There have been 125 violent deaths in the capital so far this year, the largest number in a calendar year since 2009, when there were 131.
In 2018 there have been 72 deaths involving a knife, 13 involving a gun, one involving a knife and a gun and one a crossbow.
Just over a third of victims, 44, were aged 16 to 24, of whom 10 were shot, 32 were stabbed and one was killed in an attack involving a knife and gun.
Twenty-five of the victims were aged 19 and under, six of whom were shot and 16 of whom were stabbed.
Earlier this month Metropolitan Police Commissioner Cressida Dick insisted the tide was turning against knife and gun offences, although she admitted it would take time to tackle the 180 violent gangs in London who drag children into crime.
The force released figures that showed in September, October and November this year there were 176 fewer victims of knife crime with injury aged under 25 than in the same three months in 2017, a 31% reduction.