The "mindless and disgusting" killing of a pensioner, who was punched unconscious by two happy-slapping teenagers in front of his three-year-old granddaughter, has led a shocked detective to plead with parents to take responsibility of their children.

The fatal attack on former care worker Ekram Haque, 67, was the of culmination of a summer of escalating violence by a gang of up to 12 youths who rampaged around the Tooting and Colliers Wood area assaulting the vulnerable "for kicks".

Mr Haque had just finished Ramadan prayers outside the Idara-e-Jafferiya mosque in Church Lane, Tooting, at about 9.50pm on August 31 last year when he was punched in an unprovoked attack. He later died in hospital.

The youths, who are aged 16 and 15 and are from Tooting but cannot be named for legal reasons, pleaded guilty to his manslaughter at the Old Bailey this afternoon, and police said their parents or guardians had shown little interest in the case.

The killing took place while one of the defendants was on bail for assaulting a couple in their 70s in their home just five days earlier.

The two killers and a third teenager, who is 15 and also from Tooting, all pleased guilty to causing actual bodily harm in that incident. All three also pleaded guilty to grievous bodily harm on two other men, from Ewell and Thornton Heath, less than a minute before the attack on Mr Haque.

CCTV of the attack on Mr Haque showed the youths hitting him so hard "they nearly fall over themselves with the force they use".

Police said the three were part of a group of 10 or 12 that had committed at least a dozen similar attacks – which rose in seriousness as the group got braver.

Detective John McFarlane of the Metropolitan Police said: "We have an elderly man coming out of a place of worship who is attacked in front of his granddaughter for fun, for kicks.

"It is mindless and disgusting, absolutely disgusting.

"I don't like the phrase happy slapping, it is a very serious offence. What all victims had in common was they were vulnerable. Either young, old, infirm, and were hit from behind with no chance to take avoiding action.

"We keep hearing kids say they want respect but these kids had no respect for anybody."

He said parents had to shoulder the responsibility.

"We have community engagement, safer neighbourhood teams, social workers and a whole raft of things, but some people just don't want to engage," Detective McFarlane added.

"But parents need to take responsibility for their children. They need to know where their kids are, what they are doing.

"We will always prosecute offences of violence, this behaviour is incredible and we won't tolerate it, we want to stop it. Our sympathies go out to Mr Haque's family."

Detective Sergeant Richard Reynolds added he had not seen "the parents of [one of the defendants from start to finish. The parent of another I saw for the first time in court yesterday".

Five days before the attack two pensioners were in their home and asked a group of youths to move from outside their house.

DS Reynolds said: "The gang walked into the house before the two people, in their 70s, had time to react. They were assaulted in the hallway.

"They were quite upset and distraught. They have lived in the area for a long time and this was the first trouble they had."

After Mr Haque's death police looked at one defendants' phones, which revealed happy slapping videos around the Tooting and Colliers Wood area - some of which had a voiceover introducing each attack as "a Lane Gang production".

DS Reynolds stressed the gang was not serious and organised but said the unchecked behaviour was troubling.

"Some people go and play football, others do this. What are their values?," he said.

"The videos go from them slapping someone and running away to punching them as hard as they physically can, so hard they nearly fall over themselves with the force they use."

He added: "I don't think they can comprehend what they are doing. They have no reference to the consequences."

Residents living near the killers "say it is now a lot quieter and a better place to live", he added.

Speaking from near his dad's hospital bed before he died, Mr Haque's son, Arfan, a consumer law advisor, said the family was completely devastated.

"It was an attack on a defenceless man outside a church of god with his young granddaughter," he said.

"The point is the mindless violence."

He said his dad was a kind and loving person who would go out of his way to help people and the wider Tooting community where he lived.

He added his daughter, Miriam, was "very shaken".

"She has given a vivid account of what happened," he said.

Mr Haque had planned to take his granddaughter on holiday to Pakistan and Australia.

The three teenagers will be sentenced on July 23.