Police are investigating four killings in London in just over 24 hours this weekend.
The latest victim, an 18-year-old man, was stabbed during a fight in Sutherland Walk, Walworth, in the early hours of Sunday morning.
Riot police were also called in to deal with fight at a street party in Manor House, north London, which left a man critically ill in hospital.
Researchers have suggested that warmer weather may contribute to increases in violent crime. Last year, a senior officer warned that heatwaves increased pressure on police because the public “react very strangely” to the conditions.
The Metropolitan Police said in a statement: “Bearing down on violent crime on the streets of London continues to be a top priority.
“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. This information could save lives in the future.”
In the first of the four killings, pregnant Kelly Mary Fauvrelle, 26, was stabbed to death at her home in Thornton Heath during the early hours of Saturday morning.
Her baby was delivered by paramedics at the scene of the attack and remains in a critical condition in hospital. Two men have been arrested on suspicion of murder.
Later the same day, a 54-year-old man died in hospital after being assaulted in the street in Coldharbour Lane, Brixton, shortly before 6pm.
Two men aged 62 and 66 were arrested on suspicion of murder.
The incident has been referred to the Independent Office for Police Conduct because the victim was placed under arrest before being taken to hospital. The force said it was “standard procedure”.
Then just after 11pm, officers were called to the scene of a stabbing in East Ham, Newham.
A man believed to be in his late 20s was pronounced dead at the scene near the junction of Ron Leighton Way and Wakefield Street.
The fourth victim admitted himself to a south London hospital with stab wounds in the early hours of Sunday morning and was pronounced dead at 7.23am.
Police believe he was attacked during the fight in Sutherland Walk, Walworth. Officers were called to the scene at 4.51am but could not find a victim.
Three men have been arrested on suspicion of attempted murder.
Detective Chief Inspector Neil John said: “A number of arrests have been made but we urge anyone who witnessed the incident to contact us immediately to tell us what they know.
“An 18-year-old has lost his life in violent circumstances and we are working hard to establish exactly what happened.”
Police are also investigating a stabbing near Manor House tube station in Hackney, north London, which left a man in a critical condition in hospital.
Officers were called to reports of a violent rave at the Woodberry Down Estate but were pelted with bottles and other missiles.
Specialist riot police and a helicopter were called in to deal with what was described as “spontaneous outbursts of disorder”.
A 39-year-old man was also left in a life-threatening condition after being attacked by a moped rider in a busy street in north London.
He was found unconscious in the street when police were called to reports of a fight in Stoke Newington Road at around 2.24am on Sunday.
Detective Constable Amy Harrison said: “The incident took place outside a busy bar and takeaway, where there were many people present enjoying a night out.
“I know that a lot of people witnessed this incident and filmed the incident on their mobile phones. I need those people to get in touch and speak to me urgently.”
Police have appealed for witnesses and information in relation to the attacks. Contact officers on 101 or call Crimestoppers anonymously on 0800 555 111.