Update 8.50pm: A man has been arrested on suspicion of the murder of a one-year-old boy and the attempted murder of a girl believed to be his twin sister, Scotland Yard said.
Bidhya Sagar Das, 33, was wanted by police in connection with an incident at a flat in Wilberforce Road, near Finsbury Park, north London.
Both toddlers were discovered with critical injuries at the address on Saturday night and were taken to an east London hospital, where the boy died in the early hours of Sunday morning.
Scotland Yard said Das was arrested in the Hackney area at 7.15pm on Sunday.
He has been taken to an east London police station where he remains in custody.
The girl remains in hospital in a critical condition.
Update: 3pm The suspect has been named as Bidhya Sagar Das, 33.
Scotland Yard has appealed for the public's help in tracing the suspect.
Earlier: A major search for a "suspect" in a London park is under way after a one-year-old boy died and a girl of the same age was left in a critical condition.
Officers were called to reports of two children injured in a flat in Wilberforce Road, near Finsbury Park, in north London at 11.10pm on Saturday, Scotland Yard said.
Both toddlers, believed to be twins, were found with critical injuries and were taken to an east London hospital where the boy died in the early hours of Sunday morning.
The girl remains in a critical condition.
The Metropolitan Police said: "Next of kin are aware. Formal identification awaits and a post-mortem examination will be scheduled in due course."
They added that inquiries were under way to "establish the full circumstances of the incident" and said that no arrests had been made.
The National Police Air Service helicopter tweeted that it was assisting "officers searching Finsbury Park for a suspect".
Assisting @MPSHackney officers searching Finsbury Park for a suspect.— NPAS London (@NPASLondon) March 19, 2017
A police car was parked outside the white three-storey building where the children are believed to have been injured.
Police officers have been 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 Menea, 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 Menea described the mood inside the property as "tense".
"It's really tense," he said. "There's police coming and going all the time."