Four men between the ages of 17 and 20 have died between New Year's Eve and the early hours on Monday as a result of injuries inflicted by knife attacks.
Scotland Yard has confirmed a fifth man is also in critical condition following another stabbing incident on the first day of January.
The four incidents happened in north, east and south London, and none are being treated as linked.
Five males have been arrested in connection with one of the murders, which took place in Enfield in the morning of New Year's Eve.
The other attacks took place in Tulse Hill, West Ham and Old Street.
"There were thousands of officers on duty across London yesterday and throughout the night, all determined to play their part in keeping the public safe," Met Police Commander Neil Jerome 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."
Commander Jerome thanked Londoners and tourists for co-operating with police during New Year celebrations, but added "there were small groups who chose to engage in disorder and violence, and whose actions have had utterly tragic consequences".
"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," he said.