Six males have been arrested after four people, including two teenagers, were stabbed to death in unrelated attacks across London in the space of 24 hours.
A 17-year-old boy and two men, aged 18 and 20, attacked on New Year's Eve, along with a 20-year-old man knifed in the early hours of the following morning, all died as a result of their injuries, while a fifth victim is critically ill in hospital.
Scotland Yard said a 19-year-old man has been arrested on suspicion of murder over the New Year's Day stabbing at a house party in Islington, north London.
Five males, aged between 17 and 21, have been arrested in connection with the death of the 18-year-old in Enfield, north London.
Metropolitan Police Commander Neil Jerome said four deaths in such a short period was unusual.
"There were small groups who chose to engage in disorder and violence, and whose actions have had utterly tragic consequences," 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.
"As we begin these investigations and the provision of support to four grieving families, days like this serve to reinforce the determination of the Metropolitan Police to tackle knife crime and violence on our streets; especially offences committed by and against young people.
"I would urge anyone with information about these attacks, and more generally about anyone who routinely carries a knife, to urgently pass that information to police or Crimestoppers.
"If you've hesitated to do so before, then these four tragic deaths at the start of a new year may persuade you to act now to make London safer for your family and friends in 2018."
Sunday's stabbings - at 11.30am in Larmans Road, Enfield, 7.35pm at Memorial Avenue in West Ham, east London, and 10.40pm in Norwood Road in Tulse Hill, south London - took the number of people stabbed to death in the capital last year to 80, Scotland Yard confirmed. Of those, 18 involved victims under the age of 19.
The New Year's Day stabbing occurred at a house party in Bartholomew Court, Old Street, Islington.