A six-year-old boy who was "thrown" from the 10th floor of the Tate Modern art gallery was a French national visiting London with his family.
The child was found on a fifth-floor roof after he fell from the viewing platform on Sunday and is in a critical but stable condition, the Met said.
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 custody.
The Met Police said it was treating the "truly shocking incident" as an "isolated event with no distinct or apparent motive".
Writer Mark Welte was on the observation deck with a friend when he heard a commotion and what he described as a loud "primal scream".
"Someone said someone had thrown a child over. I leapt up and looked over the rail and I did indeed see a child down below there," he said.
"The child's mother then tried to climb the rail. I restrained her and pulled her back."
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.
"There is no link between the victim and male arrested," Det Ch Insp John Massey said.
The force said the child was "no longer in a life-threatening situation" but his family was waiting for more news about his condition.
Det Ch Insp Massey added that officers were keen to speak to anybody who "witnessed a male whose behaviour seemed out of place, suspicious or worrying, in the hour or two before the incident in or near the gallery".
The Tate said it was "working closely with the police" and "all our thoughts are with the child and his family".
A spokeswoman for the gallery added the Tate was open on Monday but the viewing platform would remain shut throughout the day.
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".
BBC journalist Olga Malchevska, who was on the balcony with her son, said she heard a scream then saw some pushing.
"I just felt like something is going on, I should take my child out of there immediately and we tried to go towards the exit," she said.
She added: "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.