A teenager has been arrested on suspicion of attempted murder after a six-year-old boy was flung five floors off a viewing platform at the Tate Modern art gallery in London.

The young boy is fighting for his life after falling five floors – from the 10th floor to the fifth floor roof – about 2.45pm (local time) on Sunday.

He was treated at the scene before being flown to hospital, where he is in a critical condition.

A 17-year-old male suspect was held down by visitors to the 10th-floor viewing platform after the incident, the Met Police said.

They do not think the teenage suspect knew the six-year-old.

Nancy Barnfield, 47, of Rochdale, was at the 10th-floor viewing gallery with a friend and their children when her friend heard a “loud bang”.

Ms Barnfield turned around and saw a woman screaming, “Where’s my son, where’s my son?”.

Visitors quickly gathered around a man who was nearby, she said.

She said the person who was restrained by the public until police arrived “just stood there and was quite calm”.

BBC journalist Jonny Dymond, who was in the gallery when the incident occurred, said visitors were funnelled into a main hall while all exits were closed.

A spokeswoman for the gallery said: “Tate is working closely with the police to help with their investigations.”

“All our thoughts are with the child and his family.”

An open terrace and rooftop bar sit atop the Tate Modern, which is one of London’s most popular tourist attractions.

-with AAP