A security guard has been stabbed to death and three others were injured after they tried to stop a group of men trying to get into a New Year's Eve party in London's Park Lane.
Police and ambulances were called to the property at around 5.30am on Tuesday in response to reports of violence.
The victim is believed to be in his early 30s but has not yet been identified.
Two other men aged 37 and 29 and a 29-year-old woman were also found with stab injuries and treated at the scene before being taken to hospital but their injuries are not life-threatening.
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."
He added: "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."
Anyone with information is asked to contact police.
In a separate incident in the capital, a man was arrested after a woman was stabbed to death in the early hours of New Year's Day.
Police found the victim with stab wounds at a property on John Ruskin Street, in Camberwell, south London, at 4.20am on Tuesday.
The woman, in her early 30s, was pronounced dead at 4.50am.
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A 34-year-old man was arrested at the scene on suspicion of attempted murder and is being kept in custody at a south London police station.
The Metropolitan Police Homicide and Major Crime Command are appealing for witnesses.