Four young men were stabbed to death in separate attacks on New Year's Eve, marking one of the worst days of knife crime in the capital.
The attacks across London, which are being treated as unrelated, mark the end of a year which has seen knife and gun crime on the rise and brings the total number of people murdered in the city to 80.
One of the victims was 17 and another 18, bringing the total number of teenagers killed in London last year to a decade high of 26.
It comes just days after Cressida Dick, the Metropolitan Police Commissioner, suggested budget cuts were to blame for rising knife crime, saying that this area is where she would focus any extra resources.
The deaths have renewed calls from campaigners for tougher sentences for those carrying knives, and will be seen as a setback to London Mayor Sadiq Khan and his aim to get a stranglehold on the year-on-year increase in knife crime.
The day of violence began at 11:30am on 31 December, when an 18-year-old man was stabbed Enfield, north London, and taken to hospital where he later died. Five men have since been arrested.
At around 7.35pm a 20-year-old man was fatally stabbed in West Ham, east London.
At 10.40pm a 17-year-old college student died died from a single stab wound in Tulse Hill, south London after an "altercation" on a bus.
In the early hours of New Year's day a 20-year-old man was stabbed to death in Old Street, east London. A second man in his 20s was taken to hospital suffering critical injuries.
Four separate murder investigations have been launched by detectives from the Homicide and Major Crime Command.
Commander Neil Jerome, of the Met's Territorial Policing Command, said four deaths in such a short period was "unusual", adding: "We have certainly seen in recent months an increase in knife crime."
He said: "It is heart-breaking that, at a time when so many of us are contemplating what lies ahead in 2018, four families are dealing with the grief of losing a loved one to senseless violence and the callous use of knives as lethal weapons..
"I can assure Londoners - and the families and friends of the four males killed and the fifth who remains in hospital - that detectives will work tirelessly to bring to justice those responsible for such despicable acts of violence."
The latest figures from the Official for National Statistics show that nearly 37,000 knife crimes were committed in England and Wales in the 12 months to the end of June 2017.
This is the highest number since 2011, and 7,500 more blade offences than the same period in the previous year.
Mr Khan said: "The police were out in force helping people enjoy our city's fantastic celebrations last night, and I thank them. However, It is horrific that knife crime has claimed another four young lives as the New Year begins.
"We will work tirelessly in 2018 and beyond to stamp out this scourge.
He said that the police had assured him that "everything is being done to bring to justice those responsible for these appalling acts of violence".
Ann Oakes-Odger, who founded campaigning charity KnifeCrimes.Org after the death of her own son, said that deaths showed that more needs to be done.
"It is shocking, we are not even into the new year and that is four more families devastated," she said.
"We can't educate our way out of this problem, we do need robust sentencing for people carrying knives.
"Around the country we have seen an awful lot of cut backs and it is not surprising that the problem is getting worse. It is utterly, utterly terrible.
Separately, two men are critically ill after five people suffered stab or slash wounds in a fight at a Great Gatsby-themed New Year's Eve party in Sheffield.