Four young men were stabbed to death in separate attacks during New Year's Eve celebrations in London.
The Metropolitan Police are investigating the murders of two teenagers and two 20-year-olds this morning as the capital's knife crime epidemic shows no sign of abating.
A fifth man, in his 20s, is in a critical condition after also being stabbed at one of the incidents.
Met Commander Neil Jerome described the deaths as "heart-breaking".
"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," he added.
A 20-year-old man was fatally stabbed in Memorial Avenue, West Ham, just after 7.30pm on 31 December.
A 17-year-old then lost his life to a blade near Tulse Hill Station, south London, at around 10.40pm.
Then, just half an hour before midnight, an 18-year-old man was stabbed in Larmans Road, Enfield. He later died in hospital.
At around 2am on 1 January, a 20-year-old man was fatally stabbed in Old Street, East London, where a second man was badly wounded and taken to hospital.
Four separate murder investigations have been launched by detectives from the Homicide and Major Crime Command.
"As we begin these investigations and the provision of support to four grieving families, days like this serve to re-enforce the determination of the Metropolitan Police to tackle knife crime and violence on our streets; especially offences committed by and against young people," Jerome said.
He added: "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.
"I am grateful to Londoners and visitors to the capital who co-operated with police – both at major celebrations in Westminster and at thousands of organised and private events across London - to enable the vast majority of people to enjoy New Year's Eve.
"However, there were small groups who chose to engage in disorder and violence, and whose actions have had utterly tragic consequences."
Knife crime offences in England and Wales rose by 24% in the year ending March 2016, according to the Office for National Statistics.
Anyone with information relating to the murders can call police on 101. To remain anonymous call Crimestoppers on 0800 555 111 or contact them via the Crimestoppers website: crimestoppers-uk.org.