A member of the public said he wanted to protect a young man who later died in custody but lost confidence when scores of others stood by and ignored his cries for assistance as he and a police officer detained the individual, an inquest has heard.
The man, who has been granted anonymity, described a chaotic scene as he walked into the convenience store in Dalston, east London, in the early hours of July 22 last year.
Rashan Charles, 20, was being detained by a Metropolitan police constable, referred to during the inquest at St Pancras Coroner's Court as BX47.
The court, which had a huge black curtain barring the public and press from seeing the witnesses, the jurors, the coroner and counsel, was shown video footage from body-worn videos on both BX47 and a second officer BX48.
Giving evidence, Witness 1 spoke frequently of confusion at the scene.
He said he had gone into the shop around 1.30-1.45am when he saw a police officer who he described as "sweating" and being in "shock", and offered to help.
He helped restrain Mr Charles, placing his right hand behind his back to be handcuffed by BX47 and was shown, in video footage of the incident, straddling the young man, with one hand between his face and the floor, and the other hand on his face trying to open his mouth.
Witness 1 shouted to onlookers for anything, even a chocolate bar, to help wedge open his mouth.
Footage of the incident played to the court showed the witness pinching Mr Charles's nose and placing a finger in his mouth.
Witness 1, who had received a day of emergency medical training several years earlier, said he was just trying to make sure Mr Charles could breathe.
He also insisted he had volunteered to help protect Mr Charles, as well as assist the officer.
"I offered my assistance to defuse any tension between the two (Mr Charles and the police officer)," Witness 1 told the inquest.
"As I walked in, they both seemed to be in an awkward position on the floor and I lent an extra hand.
"I wanted to assist both. It was not mainly for the police officer, it was also to assist Rashan."
Witness 1 was heard telling Mr Charles in the video footage, "just relax".
He told the inquest: "I just wanted to show Rashan I am also there to help him and let the officer know that as well as assisting you, I am also there to protect Rashan should you get heavy handed with him."
He also spoke of losing confidence when a large number of onlookers did not try to help.
"I could not understand why there was so many people present and no one was lending a hand which would have been so vital at that time," he continued.
Witness 1 said when BX48 arrived at the scene, the officer "took better control of the situation".
Mr Charles died in hospital in the early hours of July 22 last year, shortly after he was chased by the police officer into the shop and apprehended.
Witness 1 started his evidence on Wednesday by offering his sympathies to Mr Charles's family.
Earlier, Coroner Mary Hassell granted a request by media to release video footage of the incident.
But she agreed with a request from Mr Charles's family to release a short amount of footage and not a section where paramedics worked to try and save his life.
At the start of Wednesday's hearing, Ms Hassell told the inquest the jury now consisted of 10 people, after one of the original 11 was excused following the sudden illness of a family member the previous night.
The hearing continues.