A security guard has been stabbed to death and three others injured as they tried to stop a group of men from gaining entry to a New Year's Eve party in London's West End.
Emergency services were called to the property on Park Lane in Westminster at around 5.30am on Tuesday following reports of violence.
The victim, believed to be in his early 30s, has not yet been identified however his next of kin are in the process of being informed.
The Metropolitan Police said two other men - aged 37 and 29 - and a 29-year-old woman were also found with stab injuries.
They were treated at the scene before being taken to hospital. Their injuries are not believed to be life-threatening.
It came after another New Year's Day stabbing in Camberwell, London, in which a woman, aged in her early 30s, was killed at a residential address around 4.20am.
A 34-year-old man was arrested on suspicion of attempted murder and released on bail. Formal identification of the victim is yet to take place and next of kin have not been informed.
Detectives are also investigating a shooting at a nightclub in Hackney, east London, in which a woman was shot in the leg.
Paramedics treated the victim, who is believed to be aged in her mid-20s, for a leg wound. She was then taken to hospital and her injuries are not thought to be life-threatening.
Commenting on the earlier incident on Park Lane, Detective Chief Inspector Andy Partridge said: "The incident took place outside a venue that was hosting a private party.
"I would like to hear from anyone who was present at that event and witnessed this incident.
"A number of witnesses have already been spoken to, but it is clear many others who were present are yet to be traced, some of whom may have captured the attack on their mobile phones.
"It appears the deceased and the two injured males were working as security staff for the party, they were attacked by a group of males who were outside trying to enter the venue.
"The female was also attacked as she assisted the security staff in closing the front door of the premises.
"This shocking incident happened just hours into a New Year, the area was particularly busy with people out celebrating and making their way home from various parties and organised events."
No arrests have yet been made and anyone with information is asked to contact police.