An eyewitness to the police-custody death of George Floyd described the “tremendous pain” of Floyd’s final moments as he drooled and gasped for air — saying he called 911 on the scene “because I believe I witnessed a murder.” 

“I was totally scared for my safety and people around me,” Donald Williams, who is also a competitive mixed martial arts fighter, testified during the second day in the murder trial of ex-Minneapolis cop Derek Chauvin, where prosecutors played his 911 call.

“I felt the need to call the police on the police,” he said.

As the 911 call played to the court, an emotional Williams wiped away tears.

“Officer 987 killed a citizen in front of a Chicago store,” Williams, 33, said during the chilling May 25 call, referring to Chauvin’s badge number.

“He just pretty much just killed this guy that wasn’t resisting arrest. He had his knee on the dude’s neck the whole time, Officer 987.”

“The man stopped breathing. He wasn’t resisting or nothing,” Williams told the dispatcher. “He was already in handcuffs.”

“I don’t even know if he’s dead for sure,” he continued.

“But he was not responsive when the ambulance just came and got him. And the officer that was just out here left, the one that actually just murdered the guy.”

Asked about witnessing Floyd’s death, Williams testified that “you could see that he was going through tremendous pain, and you could see it in his face.”

“You can see his eyes rolling back in his head and him having his mouth open, wide open, slowly with drool and slobber and dryness in his mouth,” he said. “And you could see that he’s trying to gasp for air, trying to be able to breathe as he’s down there, trying to move his face side to side.”

“I’m assuming to gasp for more air,” Williams added. 

Chauvin, 45, is charged with second-degree murder, third-degree murder, and second-degree manslaughter in Floyd’s May 25 death.

The former cop faces up to 40 years in prison if convicted on the most serious charge.

During the testy cross-examination, Williams — who appeared to wipe away a tear at one point during his testimony — became combative with Chauvin attorney Eric Nelson, who noted that the bystander “grew angrier and angrier” and cursed at the then-cop.

“You called him a bitch,” Nelson said.

“If that’s what you heard in the video, yes I did,” Williams answered.

At another point during the contentious cross-examination, Nelson asked, “do you recall saying ‘I dare you to touch me like that, I swear I’ll slap the f–k out of both of you.”

“Yeah, I did. I meant it,” Williams replied. “You can’t paint me out to be angry.”

“You called him a bum at least 13 times,” Nelson asked at another point.

“That’s what you count in the video?” Williams shot back.

“That’s what I counted,” the lawyer said.

“That’s what you got, 13,” Williams replied. 

Nelson also noted that Williams did not know that cops had already called an ambulance.

Viral video of the deadly encounter shows Williams berating police as Chauvin presses his knee pressed on Floyd’s neck for nearly nine minutes — despite repeated pleas from bystanders, including the martial arts fighter, that he let Floyd up.

Floyd can be heard pleading that he couldn’t breathe and calling for his mother before he stopped moving and ultimately died.

“I did call the police on the police,” Williams said under questioning by Minnesota Assistant Attorney General Matthew Frank, the lead prosecutor on the case.

“I believe I witnessed a murder. I felt the need to call the police on the police.”

“I just never seen that and I felt like I should speak on it,” Williams said. “I was trying to keep my professionalism and make sure that I speak out for Floyd’s life because I felt like he was in very much danger.”

Williams said he tried to confront cops at the scene “for fear of myself and fear of the people around me and just trying to be able to talk to the officers and let them know that this should not happen.”

The highly anticipated trial got underway Monday after three weeks of jury selection — with the case being heard under heavy security and fortifications around the courthouse, Minneapolis police precincts, and other official buildings.

Release of the video last year sparked worldwide protests and rioting.