A youth, 17, has been arrested on suspicion of attempted murder for throwing a child, six, off the 10th floor viewing balcony of the Tate Modern.
The boy was taken to hospital by air ambulance after being found on the fifth floor roof about 2.45pm on Sunday. He is in a critical condition, the Metropolitan police said.
“There is nothing to suggest that he is known to the victim,” police said.
Gallery visitors were kept inside as officers dealt with the incident and the Tate was closed for the rest of the day.
Some visitors posted on social media that the venue had been sealed off.
The Tate Modern is one of London’s most popular venues, attracting about 5.9 million visitors a year and often thousands of visitors a day.