A six-year-old boy is in a critical but stable condition after he was “thrown” from the 10th floor of the Tate Modern art gallery in London.
The child was found on a fifth floor roof after he fell from the viewing platform on Sunday afternoon.
A witness said they heard a “loud bang”, before seeing a woman scream: “Where’s my son, where’s my son?”
A 17-year-old boy arrested on suspicion of attempted murder remains in police custody.
The Met Police said there was nothing to suggest he was known to the victim.
A force spokesman added the child was “no longer in a life-threatening situation”.
Emergency services were called to the gallery, based in Bankside near London Bridge Station, at about 14:40 BST.
“We treated a person at the scene and took them to hospital,” a London Ambulance Service (LAS) spokesman said.
The Tate said it was “working closely with the police” and “all our thoughts are with the child and his family”.
Admin worker Nancy Barnfield, 47, of Rochdale, was at the 10th floor viewing gallery with a friend and their children when her friend heard a “loud bang”.
Ms Barnfield said she 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”.
Olga Malchevska, who was also on the 10th floor, said she heard a scream and then headed to the exit with her child.
“Some people started panicking. We got to the packed lift. People there were saying that one boy threw another one from the balcony. We were all shocked.”
Stuart Haggas said he saw emergency crews moving along the roof between the gallery’s Turbine Hall and its recent extension.
“They were carrying a stretcher with someone on it, plus a second stretcher was waiting by the door,” he said.
BBC correspondent Jonny Dymond, who was also there, said visitors were “funnelled towards the main Turbine Hall and the exits were all closed”.
“There were quite a lot of families with children, and security guards told us we couldn’t leave,” he said.
“There were at least two fire engines, 10 police cars and an incident control unit. Parts of the exterior of the building were taped off.”
The Tate Modern opened in the disused power station on the River Thames in 2000, while the extension with the viewing platform was opened in 2016.
It was the UK’s most popular tourist attraction in 2018 with 5.9 million visitors, according to the Association of Leading Visitor Attractions.