A one-year-old boy has died and a girl also aged one is fighting for life in hospital after they were found injured at a flat in Finsbury Park.
Murder detectives are investigating after the two children were found by police with critical injuries at a flat in Wilberforce Road.
Scotland Yard said officers and paramedics were alerted to the incident at 11.10pm on Saturday.
Neighbours said a woman was heard frantically shouting for help.
Medics rushed the injured children, believed to be twins, to a hospital in east London.
The boy was pronounced dead shortly before 1am in hospital, while the girl remains there in a critical condition.
A major police search is under way for a potential suspect.
On Sunday morning, a police car was parked outside a white three-storey building where the children are believed to have been injured.
Police officers were visiting and searching properties along the tree-lined street.
A woman living opposite the building, who gave her name as Gui Gui, said she heard a woman shouting late last night and opened the window to offer help.
"I was watching TV," she said. "I heard someone was shouting. She kept on shouting. I do not know what she was shouting.
"I opened the window and I asked her 'Can I help you, can I call the police for you?' She said, 'My kids'."
The neighbour said she later saw two young children being carried out of the building, with one being held very close to a member of the emergency services.
Mihai Manea, 29, who lives on the second floor of the building, said the children were twins.
They lived on the top floor with their Romanian mother and Indian father, he said.
The father worked at the nearby Pembury Hotel as a receptionist, where Mr Menea also works, until a few days ago, he said.
Mr Manea described the mood inside the property as "tense".
"It's really tense," he said. "There's police coming and going all the time."
Detectives from the Homicide and Major Crime Comman are investigating.
Police have not yet released details of how the children may have sustained their injuries.
A Met Police spokesman said: “Next of kin are aware. Formal identification awaits and a post-mortem examination will be scheduled in due course.”
He added that enquiries were underway to “establish the full circumstances of the incident” and said that no arrests have been made.
A National Police Air Service helicopter was assisting Hackney officers with a search of the Finsbury Park area in the early hours of Sunday morning.
A spokesman said they were "assisting officers from Hackney after two children were found seriously injured at an address in Wilberforce Road."