A teenager was arrested on Sunday after allegedly throwing a 6-year-old boy from a 10th-floor viewing platform at London's Tate Modern gallery, police said.

The boy was airlifted to a hospital and is in a critical condition, while the 17-year-old male suspect is being held on suspicion of attempted murder.

The Tate Modern, a leading contemporary art museum on the banks of the River Thames, was initially locked down but visitors were later allowed to leave.

Several members of the public gave witness statements.

"Police were called at around 2:40 pm (1340 GMT) on August 4 to reports of a young boy thrown from the 10th floor viewing platform of the Tate Modern," a police statement said.

"The 6 year-old victim was found on a fifth floor roof. He was treated at the scene and taken to hospital by London's Air Ambulance. 

"The boy's condition is critical; his family are being supported by police," the statement said.

Police said the suspect "remained with members of the public on the 10th floor viewing platform. There is nothing to suggest that he is known to the victim."

"The teenager was arrested by police officers on suspicion of attempted murder and taken into police custody," the statement said.

The Tate Modern is Britain's top visitor attraction. The museum saw 5.87 million visitors in 2018, according to the UK's Association of Leading Visitor Attractions.