People walk past the Tate Modern gallery in central London. Image Credit: AFP

London: A six-year-old French boy thrown off a 10th-floor viewing platform at London's Tate Modern gallery has suffered a broken spine, legs and arm, a court heard on Tuesday.

The boy, who was visiting London with his family when the incident happened on Sunday, also has bleeding to the brain, prosecutor Sian Morgan told the court at a hearing for the 17-year-old alleged assailant.

The British teenager, who was arrested at the scene, appeared at Bromley Youth Court in southeast London on Tuesday after being charged with attempted murder.

The boy, who cannot be named for legal reasons, spoke only to confirm his name and address, an AFP reporter witnessed.

Wearing a grey jumper and tracksuit, he was white, had short hair and was unshaven.

Police have said the victim's condition is no longer life-threatening.

"The injured boy remains in a stable but critical condition in hospital with his family, who continue to be supported by officers," a police statement read.

They appealed for anyone who saw the incident to come forward.

The contemporary art museum on the banks of the River Thames, Britain's most popular visitor attraction, was evacuated after the incident on Sunday but reopened on Monday.

However, the Tate Modern said the viewing platform was closed. The boy fell from there onto a fifth-floor roof and was airlifted to hospital by helicopter.

The alleged assailant is due to appear in court again on Thursday.