London's Tate Modern gallery was evacuated on Sunday after a child fell "from a height" and was airlifted to hospital.
A teenager was arrested over the incident, police said, without giving any details of the child's condition.
The doors of the contemporary art museum were initially locked as emergency services gathered at the site on the banks of the River Thames, an AFP journalist said.
"We are evacuating people as we speak," said a staff member in the membership department, who declined to give her name.
She told AFP there had been "quite a few witnesses".
"Officers are in the vicinity of the Tate Modern following an incident this afternoon," a brief statement from the Metropolitan Police said.
"A child fell from height and is being taken to hospital by London's Air Ambulance. We await an update on his condition.
"A teenager has been arrested in connection with the incident." The Tate Modern is Britain's top visitor attraction, with 5.87 million visits in 2018, according to the Association of Leading Visitor Attractions.