An autistic teenager could be seen laughing moments after he threw a six-year-old boy from the Tate Modern viewing platform, a court heard, telling onlookers: “It’s not my fault, it’s social services’ fault.”

Jonty Bravery spent more than 15 minutes stalking potential victims at the London tourist attraction before fixing on a young visitor who had briefly left his parents’ side, the Old Bailey heard.

The well-built 17-year-old, from Ealing in West London, was said to have “scooped (the victim) up and, without any hesitation, carried him straight to the railings and threw him over”.

The victim, who cannot be identified because of his age, fell around 100ft (30m) to a platform below, while disbelieving witnesses, including the boy’s parents, challenged Bravery.

The victim’s father originally thought the incident was “a joke” until he saw his son’s severely injured and bloodied body below.

The child’s mother became “increasingly hysterical” and tried to climb over the railings to get to her son several stories below, but was held back by staff, the court heard.

Bravery, who is now 18 and admits attempted murder, was said to “have a big smile on his face” and told the boy’s father: “Yes I am mad.”

The court heard he was under one-on-one supervision with Hammersmith and Fulham Social Services at the time of the attack on August 4, but was allowed to go out unaccompanied for four-hour periods.

Prosecutor Deanna Heer told Bravery’s sentencing hearing on Thursday that CCTV captured the incident, then showed the defendant backing away from the railings.

She said: “He can be seen to be smiling, with his arms raised. At one point, he appears to shrug and laugh.”

The court heard Bravery made his way on foot to the Tate Modern, arriving at 2.16pm, having previously scoped out the Shard – Britain’s tallest building – but was unable to afford a ticket.

Witnesses at the Tate said he was “behaving in an unusual way” and was seen to look over the railings near where he would later throw the boy.

Two women, visiting the tourist attraction with their two sons aged 11 and eight, saw the defendant “smiling at the children”, the court heard.

The court was told the victim and his family – on holiday from France – arrived at the Tate Modern viewing platform at 2.32pm, having spent the day sightseeing and having a picnic by the river.

CCTV then caught Bravery turning towards the victim’s family, with the boy skipping a little way away from his parents.

Ms Heer said: “As (the boy) approached, the defendant scooped him up and, without any hesitation, carried him straight to the railings and threw him over.

“The CCTV footage shows (the boy) falling head-first towards the ground.”

The court heard the boy suffered life-threatening injuries, and spent more than a month in hospital in the UK before being discharged to a hospital in France.

He remains in a wheelchair, and will require 100% care support until at least August 2022.

Ms Heer said: “Whether he will ever make a full recovery is not known.”

She said Bravery blamed social services when challenged by witnesses moments after the incident.

The prosecutor said Bravery “sniggered”. Asked why he had done it, Bravery said: “It’s a long story.”

He was also heard to say, “It’s not my fault, it’s social services’ fault”, with a shrug, the lawyer said.

Following his arrest, Bravery was said to have asked police if he was going to be “on the news”.

He said that he had been “seriously unhappy” recently and that he had to do anything he could to get out of his accommodation.

Ms Heer told the court: “He said he had to prove a point to ‘every idiot’ who had ever said he did not have a mental health problem that he should not be in the community.”

He said he had been planning the incident for a long time, the court was told.

Giving evidence, Dr Joanna Dow, a consultant forensic psychiatrist who works at Broadmoor Hospital where Bravery is being held, said she believed the defendant had a mixed personality disorder and struggled to manage his emotions.

She recommended Bravery be detained in hospital, rather than handed a prison sentence, so he could get treatment such as anger management and to learn social communication and interaction skills.

Bravery appeared in court via videolink from Broadmoor.

At one stage he could be seen with his T-shirt over his head, and moved from his chair to the floor.

The hearing continues, although the judge said Bravery would be sentenced on Friday.