The six-year-old child who was "thrown" from the 10th floor viewing platform of the Tate Modern was a French national on vacation with his mother, it has been revealed.
A teenager has been arrested on suspicion of attempted murder, and police spokesman told PA that the child, who landed on a fifth floor roof, is “no longer in a life-threatening situation”.
“He is critical, but stable,” the spokesman added.
They added that the victim and the 17-year-old boy who was arrested were not known to each other.
The Metropolitan Police said the child has been taken to hospital by Air Ambulance.
Following the fall, the teenager remained on the viewing platform with members of the public.
Footage shared on social media showed the Air Ambulance landing at the scene.
Admin worker Nancy Barnfield from Rochdale was in the 10th floor viewing gallery with a friend and their children when her friend heard a "loud bang".
Ms Barnfield turned around and saw a woman screaming "where's my son, where's my son?"
Members of the public quickly gathered around a man who was nearby, she said.
Ms Barnfield said: "We did not notice the mum before, we noticed her after because she was hysterical by then."
She said the person who was restrained by members of the public before the police arrived "just stood there and was quite calm".
But most visitors only discovered that someone had been injured as they tried to leave.
A group of uniformed police officers could be seen going into one of the gallery entrances at 4.53pm as the metal shutters were brought down and visitors were turned away.
Visitors reported on social media not being allowed in or out of the gallery while emergency services dealt with the incident, however, they have since been allowed to leave.
BBC journalist Jonny Dymond, who was in the gallery at the time, said visitors were funnelled into a main hall while all exits were closed.
He added: "There were quite a lot of families with children, and security guards told us we couldn't leave.
"There was at least two fire engines, 10 police cars and an incident control unit.
"Parts of the exterior of the building were taped off."
A London Ambulance service spokesperson said that after they were called at 2.40pm two ambulance crews were sent to the scene, along with the Hazardous Area Response Team and the air ambulance.
"We treated a person at the scene and took them to hospital as a priority," the spokesperson added.
A sign placed outside the gallery confirmed that it would be closed for the rest of Sunday.
A spokeswoman for the gallery said: "Tate is working closely with the police to help with their investigations.
"All our thoughts are with the child and his family."
The Tate Modern was the UK's most popular tourist attraction in 2018 after being visited 5.9 million times, according to the Association of Leading Visitor Attractions.
With the schools having broken up and holiday season begun, there are likely to have been thousands of visitors to the gallery on Sunday.