A six-year-old boy was thrown five floors from the 10th floor of the Tate Modern art gallery in central London, police said.
He landed on a fifth floor roof and was taken to hospital by air ambulance following the incident at about 14:45 BST on Sunday.
The boy was said to be in a critical condition.
A 17-year-old boy has been arrested on suspicion of attempted murder, the Met Police said.
They said the six-year-old was thrown from a viewing platform.
“There is nothing to suggest that [the arrested youth] is known to the victim,” a Met Police spokesman said.
Visitors were initially locked inside the gallery at Bankside on the South Bank.
“We treated a person at the scene and took them to hospital as a priority,” a London Ambulance Service (LAS) spokesman said.
BBC correspondent Jonny Dymond, who was at the gallery when the child fell, 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 was at least two fire engines, 10 police cars and an incident control unit. Parts of the exterior of the building were taped off.”
‘Quite a few witnesses’
A staff member, who did not want to be named, said: “We are evacuating people as we speak”.
She added that there had been “quite a few witnesses” to the boy’s fall.
As well as the air ambulance, two road ambulance crews were sent to the scene, along with the Hazardous Area Response Team, the LAS spokesman added.
The Tate Modern opened in the disused power station on the River Thames in 2000.
It was the UK’s most popular tourist attraction in 2018 with 5.9m visitors, according to the Association of Leading Visitor Attractions.
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