Bidhya Sagar Das, believed to be the children’s father, was on the run when his partner discovered that their two children had been battered with a hammer at their north London home on Saturday, reported the Daily Mail.
He was arrested on Sunday in Hackney, London, the Scotland Yard said.
The Yard said that the incident appeared to be “domestic”, although inquiries continue.
Neighbours said that the twins lived on the top floor of the building with their Romanian mother, Cristinela Datcu, and Indian-origin father.
It is believed the mother was locked in the bathroom as the children were attacked with a hammer, the Mail reported.
“When a neighbour went into the room, the boy was already dying. The boy had been hit on the head with a hammer,” said Mihai Manea, a neighbour.
The neighbour claimed Datcu was taking a shower at the time of the attack. He said: “In that time, someone has killed the child.”
The police found the children seriously injured after being called to a flat on Wilberforce Road near Finsbury Park at 11.10 p.m. on Saturday.
The children were rushed to hospital, where the boy died on Sunday and the girl remained in critical condition.
A hammer reportedly found dumped near the apartment is being examined by forensics officers.
Detective Chief Inspector Dave Whellams, of the Homicide and Major Crime Command, said: “This is clearly a tragic incident, and our thoughts are with the family of the two young children.”
“Whilst we remain in the early stages of the investigation, a man has now been arrested. I would still like to hear from anyone who may have any information regarding this terrible incident.”
“Next of kin are aware. Formal identification awaits and a post-mortem examination will be scheduled in due course.”
Following his arrest, Das was taken to a police station in East London, where he remained in custody.