British Police arrested a teenager on Sunday after a six-year-old boy was allegedly thrown from a tenth-floor viewing platform of London's Tate Modern gallery.

A woman screamed "he's my son" as a child fell from the tenth floor of the popular art gallery on Sunday afternoon after the child was allegedly pushed by a teenager, the Daily Mail reported.

The victim was airlifted to 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.

"I heard the impact and then screaming from above as a woman screamed 'He's my son! He's my son!'," an eye-witness, who had been on the fifth floor in the member's lounge, told The Mirror. "I went inside because the screaming was horrific, the boy didn't make any noise but the people from the viewing platform were screaming."

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

"The six 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."

Police said the suspect "remained with members of the public on the tenth 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 custody."

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