Derek Chauvin, the ex-police officer accused of murdering George Floyd, was "justified" in using force to pin him to the ground, a key defense witness told a US jury on Tuesday.

Defense lawyers opened their case after prosecutors wrapped up their arguments earlier in the day.

Barry Brodd, a use of force expert, told the trial in Minnesota that the accused "was acting with objective reasonableness, following Minneapolis Police Department policy and current standards of law enforcement."

"From a police officer's standpoint, you don't have to wait for it to happen. You just have to have a reasonable fear that somebody is going to strike you, stab you, shoot you."

He added that Chauvin appeared fearful of bystanders, point to the fact he had drawn his pepper spray canister during the arrest. 

Chauvin, who is white, faces charges of murder and manslaughter over the arrest last May that led to Floyd's death after he kneeled on the 46-year-old's neck for more than nine minutes.

It sparked accusations of racism, and prompted Black Lives Matter demonstrations across the US and elsewhere around the world.

What is the defense team arguing?

The cause of death of the 46-year-old Floyd is central to the case against Chauvin.

Prosecutors say he died from asphyxia, or a lack of oxygen; the defense claims his past drug use and heart condition explained his death in the circumstances.

"The evidence will show that Mr Floyd died of a cardiac arrhythmia that occurred as a result of hypertension, his coronary disease, the ingestion of methamphetamine and fentanyl, and the adrenaline flowing through his body, all of which acted to further compromise an already compromised heart," Chauvin's lawyer, Eric Nelson, said in his opening statement.

He brought up a 2019 arrest in which Floyd suffered from dangerously high blood pressure and confessed to heavy use of opioids.

Prosecutors had called witnesses who testified that keeping Floyd face down with hands cuffed behind his back, with three police officers restraining him, led to low levels of oxygen.

But Brodd told the jury: "If the officer is justified in using the prone control, the maintaining of the prone control is not a use of force. It's a control technique."

What happened during cross-examination?

Steve Schleicher, the lead prosecution lawyer, went through video clips once more of a pinned-down Floyd gasping that he couldn’t breathe and then going limp.

He asked Brodd if a reasonable officer might take the man's words at face value given that he had the weight of three officers on him at the time.

"It's possible," he responded. But Brodd again asserted that Floyd, even minutes into the prone restraint, was still "struggling" against the officers.

"Struggling or writhing?," Schleicher asked.

"I don't know the difference," Brodd responded.

The defense team's star witness also appeared to claim that if someone can talk, he or she can breathe.

"I certainly don’t have medical degrees, but I was always trained and feel it's a reasonable assumption that if somebody's [saying], 'I'm choking, I'm choking,' well, you're not choking because you can breathe," Brodd told the court.

Who else testified on Tuesday?

Lawyers for the defense called Shawanda Hill, who was in the SUV with Floyd before his encounter with Chauvin.

She was sitting in the back seat of the car when Floyd was approached by store employees who confronted him over an alleged counterfeit $20 bill.

Hill testified that Floyd "instantly grabbed the wheel" as soon as he saw an officer with a weapon.

"He was like, 'Please, please, don't kill me. Please, please, don't shoot me. Don't shoot me. What did I do? Just tell me what I did," she said.

Minneapolis Park Police Officer Peter Chang, who helped at the scene that day, said he saw a "crowd" growing across the street that "was becoming more loud and aggressive, a lot of yelling across the street."

"Did that cause you any concern?" Nelson asked.

"Concern for the officers' safety, yes," Chang replied.

It is as yet unclear whether Chauvin will take the stand himself. This could lead to a devastating cross-examination from the prosecution but might also be his best chance to directly put his case to the jury.

jf/msh (AP, AFP, Reuters)