Zahid Saeed, 38, described Mr Bayoh as his “brother” and said told a public inquiry that he was murdered at the hands of police officers,.
The 38-year-old told Angela Grahame QC, leading the inquiry in Edinburgh: “The truth is he (Mr Bayoh) was murdered in police custody.”
Mr Bayoh, 31, died while being held by officers on a police call-out to “a man armed with a knife” in Kirkcaldy, Fife, on May 3 2015.
A written statement from Mr Saeed displayed to the inquiry showed Mr Bayoh had taken drugs the night before he died and confirmed he was armed with a blade.
He was also seen with an 8-inch kitchen knife and fighting with another man hours before he was pronounced dead, according to neighbours called to give evidence.
Fighting back tears, Mr Saeed told the hearing he has been through two traumas, adding: “First one was my son who was killed, then my friend, who was murdered in police custody.
“That has caused trauma.
“The questions you should be asking is to the police officers, not me, not me.”
Lord Bracadale, chairing the inquiry, adjourned Mr Saeed’s evidence until a later date.
Mr Saeed is the father of Edinburgh toddler Mikaeel Kular, three, who was reported missing from his home in Edinburgh's Muirhouse in January 2014.
His remains were discovered days later buried in a garden in Kirkcaldy. The youngster had endured a horrific beating before dying of his injuries.
Mikaeel’s mother, Rosdeep Adekoya was later jailed for 11 years after she admitted culpable homicide.
After the case, Mr Saeed was described as being “traumatised’ at the horrific death of his son.
The inquiry also hard from Neil Morgan, who lived opposite Mr Bayoh in Kirkcaldy, who say him fighting someone in a nearby garden at about 7am.
Mr Morgan said he saw Mr Bayoh carrying a kitchen knife about 8in long.
Ms Grahame asked the witness what Mr Bayoh was doing with the knife, to which he replied: “He was tapping it on his leg, and I said to him, ‘you can’t go round with that knife, you’ll get done’.
“Then he turned around to me and he said: ‘It’s not even sharp’… and sort of poked it at my belly.”
He told the inquiry that on the morning of his neighbour’s death, Mr Bayoh “wasn’t himself” and had “starry” and “gazey” eyes.
He added: “He was calm, he didn’t seem in a rage. He was himself, but with a knife.”
Neighbour Naomi Rhodes, also giving evidence, said she was woken by shouting outside her bedroom window the morning Mr Bayoh died.
She told the inquiry she saw two males fighting, and one of them was Mr Bayoh.
When Ms Grahame asked her about Mr Bayoh’s behaviour that morning, Ms Rhodes replied: “He wasn’t being himself, he was fighting.”
The inquiry, before Lord Bracadale, continues.