A mother was heard screaming "my kids, my kids" after a one-year-old boy was killed and a girl of the same age left fighting for life in an alleged hammer attack.
On Sunday night arrested a man, believed to be the mother's partner, after a major search.
Scotland Yard said Bidhya Sagar Das, 33, was arrested on suspicion of the murder of a one-year-old boy and the attempted murder of a girl of the same age at a flat near Finsbury Park, north London.
Das was at the property where the children were injured but left before police and ambulance crews arrived.
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.
Police said that the suspected murder, at a flat in north London on Saturday night, is believed to be domestic.
Assisting @MPSHackney officers searching Finsbury Park for a suspect.— NPAS London (@NPASLondon) March 19, 2017
Mihai Menea, 29, who lives in the building said: "A neighbour heard the woman screaming in the street and went up to the room.
"He went into the room and the boy was already dead when he found him.
"I couldn't imagine something like this happening in a million years."
He believed the mother was Romanian and the father is Indian and worked as a night receptionist at The Pembury Hotel round the corner for seven years, but apparently quit his job two days ago.
He confirmed the children were twins, a boy and a girl.
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 a critical condition in an east London hospital.
The Metropolitan Police had been trying to find Das throughout Sunday, releasing an earlier statement saying: "Detectives are urgently trying to trace a man who was at the flat shortly before the injured children were found, but left before emergency services were called. The man has not been seen since this time.
"He is Bidhya Sagar Das, 33, of the address in Wilberforce Road where the injured children were found.
"If any members of the public see this man or know his whereabouts, they are urged to contact police via 999 immediately.
"The incident is believed to be domestic. There has been no arrest at this stage and enquiries continue."
The National Police Air Service helicopter tweeted that it was assisting "officers searching Finsbury Park for a suspect".
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 added: "I said what has happened and she just shouted 'my kids, 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.