Two teenagers have been found guilty at Leicester crown court of the manslaughter of Bhim Kohli after the 80-year-old was attacked in a park near Leicester in September.

A 15-year-old boy, who kicked, punched and racially abused Kohli while he was walking his dog in a park, has been found guilty of manslaughter.

A court heard that while Kohli was on his knees, the boy took off one of his slider shoes and slapped the elderly man in the face.

The incident was filmed by a 13-year-old girl, then 12, who could be heard laughing and was said to be encouraging the attack. A jury also found her guilty of manslaughter.

Jurors deliberated for six hours and 46 minutes before unanimously convicting the pair of manslaughter. The boy had also been charged with an alternative count of murder, but was found not guilty by the jury.

The teenagers, who were in the dock for the first time since their trial began, sat as the foreman of the jury returned the verdicts.

Leicester crown court heard that Kohli suffered a broken neck and three broken ribs in the assault that happened on his daily dog walk, metres from his home in Braunstone Town, Leicestershire. He died in hospital the following day.

He told a paramedic the boy had racially abused him and he had been pushed, punched and kicked.

The jury was shown footage filmed by the girl of Kohli on his knees as the boy slapped him.

Harpreet Sandhu KC, prosecuting, told the jury: “You heard that slap, you heard the slider making contact with Mr Kohli’s face. There was nothing for [the boy] to fear from an 80-year-old man who was on his knees. It was pure violence used by [the boy], pure violence encouraged by [the girl].

“[The girl] made a noise. That noise was her laughing. She laughed as [the boy] assaulted Mr Kohli.”

Sandhu said the girl had prior knowledge of what was about to happen. He said: “She knew there would be violence and she had a desire to capture it – and capturing it provided encouragement for the violence to be meted out.”

The youth had no reason at all to hit Kohli, Sandhu said, adding that the boy’s actions had been “gratuitous violence against a man who was defenceless and a man who, because he was on the ground, was in a vulnerable position”.

The court heard that the defendants, who cannot be named because of their age, were part of a group of five children in the park that evening. When they saw Kohli the girl told the group that the elderly man had tried to hit one of the boy’s friends with a stick.

Only the two defendants approached Kohli and the others ran away. One later told police they did so because they thought the boy was “either going to start on him or be mean”.

Jurors were read a letter the boy wrote while in custody in which he said he hated what he did. He wrote: “I regret it so much. I have flashbacks of that day and it just upsets me. I just want my freedom or even to go back and not do it.

“I feel like my case is evil. I ain’t that type of person. I kinda just needed anger etc releasing.”

He continued: “I feel like I have let my mum down so much. I am nervous, well scared and worried. I accept I did it and I am doing time. I am just scared about how long I have to do.”

The prosecution said at no point in the letter did the boy say he was acting in self-defence or in someone else’s defence. When told the letter would have to be disclosed, he responded: “That’s my manslaughter plea gone.”

The boy denied charges of murder and manslaughter. In a prepared statement he said he thought Kohli was about to hit the girl, so he ran over and fell on top of him.

Kohli’s family said they were heartbroken by his death, describing him as a “loving, caring person whose life centred around his family”.

In a statement released by Leicestershire police, Kohli’s daughter Susan Kohli said: “The boy, who told witnesses he had anger problems and couldn’t stop, used violence so severe that he broke three of Dad’s ribs and neck, which caused trauma to his spinal column.

“Videos of the incident were filmed and shockingly found on the girl’s phone. Dad did not deserve this, and we wouldn’t wish this pain on anyone else … Hearing the girl laugh at this assault on Dad is utterly disgusting. This sound plays over and over in our heads.

“Losing Dad in these cruel, violent and deeply shocking circumstances feels like our hearts have been pulled apart. We can’t put into words the pain we feel every day, and this has magnified during the trial.”

The judge remanded the boy into custody and granted the girl bail, with sentencing adjourned until 20 May.