Police have launched a murder investigation after a pensioner was stabbed to death and two children also suffered stab wounds.
A third child had minor injuries in the incident at an address in Tower Hamlets, east London.
A 31-year-old man who was found with self-inflicted injuries was taken to hospital where he remains in a stable condition.
He has been arrested by detectives on suspicion of murder, Scotland Yard said.
Police were called by the London Ambulance Service at 6.51pm on Sunday to reports that help was needed at an address in Huddleston Close, Bethnal Green, a Yard spokesman said.
Officers found five people suffering from various injuries. A two-year-old boy and an 11-year-old girl both had stab injuries. A 13-year-old boy was found with minor injuries, which were not stab wounds.
A 79-year-old man who had stab injuries was taken to an east London hospital where he was pronounced dead at 10.07pm. The children were taken to an east London hospital where they are in a stable condition.
The 31-year-old man is also in an east London hospital. Following treatment for his injuries he will be released from hospital and taken to a police station for questioning, the spokesman added.
Formal identification and a post-mortem examination will be scheduled in due course.
Detectives from the Homicide and Major Crime Command are investigating.
Police are not seeking any other people in connection with the incident.
One neighbour, Tasha, 37, saw the older man being treated by paramedics in an upstairs window.
"They were working on him and he seemed to be okay," she said. "The policeman carried out the baby and you could see where he had been stabbed. He had a bloodstained top. The other two children walked out."
She did not hear the initial incident but went outside after seeing police and paramedics arrive.
Other neighbours also spoke of their shock.
Helen, who has lived on the road for 23 years, said she had seen the 31-year-old man the day before the incident.
"I waved and spoke to him on Saturday. He joked with my husband about having a drive in our Jaguar. He was always smiling, a nice man."
She said the 31-year-old used to play football with her son and the family had moved to the property when it was first built 23 years ago.
She added: "There has never been any trouble. The old man was very quiet."
Ms Fatma, 33, who lives opposite the family, said there was a large extended family living at the six-bed property.
She said: "I am so shocked. They were a really lovely family, there has never been any trouble."
Of the 31-year-old man arrested by police, she said he was "quiet, very quiet".