A teenager is in custody on suspicion of attempted murder after a six-year-old boy was thrown from a 10th floor viewing platform at the Tate Modern art gallery.
The victim fell onto a fifth floor roof, and is in a “critical but stable condition” after being air lifted to hospital by London’s Air Ambulance service.
A police spokesman said he is "no longer in a life-threatening situation".
The suspect, a 17-year-old boy, remained on the platform with onlookers until the police attended at around 2.40pm on Sunday afternoon.
Police have said there is nothing to suggest that the suspect and the victim are known to each other.
Visitors took to social media, reporting that they were not allowed in or out of the space while the emergency services were dealing with the incident.
Scotland Yard said several members of the public are assisting with witness statements.
One witness, Nancy Barnfield, 47, was there with a friend and their children when her friend heard a “loud bang”.
Ms Barnfield reported that she then saw a woman screaming “where’s my son, where’s my son?”.
She added that some of the other visitors quickly gathered around and restrained a nearby man who “just stood there and was quite calm”.
A spokeswoman from the Tate said: "Tate is working closely with the police to help with their investigations.
"All our thoughts are with the child and his family."
The gallery reopened this morning after being shut down for the rest of Sunday but the viewing platform will remain closed.