hree women and a man were found stabbed to death in their family home in south east London early on Monday.
Neighbours described waking up to a “bloodbath” and “scenes of carnage” in Bermondsey.
Police were called to reports of a disturbance at a three-bedroom terraced house on Delaford Road about 1.40am.
Officers attended, forced entry and discovered four people suffering what are believed to be stab injuries inside.
Despite the efforts of emergency services, the victims were pronounced dead at the scene.
The four victims were three women aged in their mid 60s, 40s and 30s, and a man in his mid 60s, police have said.
A man in his late 20s was arrested on suspicion of murder at the address and remains in custody.
Metropolitan Police Commander Commander Colin Wingrove, speaking at the scene on Monday afternoon, said: “In the early hours of this morning, following a call from neighbours, officers forced entry into a house here in Delaford Road.
“Inside the house, they made a dreadful discovery of four people who’d been stabbed.”
He also said: “Such terrible events are rare, but this will not diminish the shock that we felt across London and beyond.
“And I can assure you this investigation will be thorough to ensure justice for the victims and their families and the community.
“I want to offer my heartfelt condolences to the family and friends of all those concerned.
“This is a deeply shocking incident and we will do everything we can to provide support to everyone affected at this difficult time.”
On Monday, four crime scene tents protected the bodies in the street where they had been given first aid as forensic officers trawled for clues.
A neighbour said: “I was watching telly and I heard screaming. It was a woman, they were terrible screams. A little later I head a helicopter and looked out and saw police everywhere. It looked like the police had guns. They forced entry to the house.”
A pensioner, whose house overlooks the horror, said: “There were loads of police and paramedics working on the victims outside. It looked like a bloodbath – a scene of carnage.
“They were my neighbours, I think. A lovely couple. I can’t believe anyone would want to hurt them.”
She added: “They have been here for years. I am so shocked – it’s a nightmare. I can’t tell you how nice these people were.”
A mother-of-two living opposite the murders added: “I woke up this morning to see four tents with police officers saying there has been stabbings. It looked like carnage. It was like something out of a film.”
Mayor Sadiq Khan said: “Heartbroken to hear that three women and a man were killed last night in a devastating incident at an address in Southwark.
“My thoughts are with the family and friends of those who have lost loved ones in this awful crime.
“An investigation is underway and a man has been arrested.”
Harriet Harman, MP for Camberwell and Peckham, tweeted it was “tragic” and that she was working with the Met and local council.
The scene is close to Millwall FC’s stadium The Den and South Bermondsey train station.
Pictures from the scene showed part of the road closed off with metal hoarding.
Scotland Yard said officers are in the process of contacting next of kin. Post-mortem examinations will be arranged in due course.
Detectives from the Met’s Specialist Crime Command are investigating one of London’s largest single homicides in recent years.
Enquiries continue to establish the circumstances, a Met spokesman said.