MILWAUKEE — Body camera footage of a fatal police shooting is expected to take center stage in the homicide trial of the former Milwaukee officer who fired the shots, opening statements on Tuesday suggested.

A Milwaukee County prosecutor described the video footage from the shooting, which has not yet been seen publicly, in chilling detail: The officer, Dominique Heaggan-Brown, had just shot Sylville K. Smith, 23, in the upper arm after a brief chase last August. The video shows Mr. Heaggan-Brown as he “stood over” Mr. Smith, then shot him in the chest at point-blank range, said the prosecutor, John Chisholm.

Mr. Smith was “on the ground, unarmed, hands up, with no place to go,” Mr. Chisholm said.

Jonathan Smith, Mr. Heaggan-Brown’s lawyer, countered that the video would prove that the officer “was well-trained in police tactics and use of force, including lethal force,” he said, citing Mr. Heaggan-Brown’s three years on the Milwaukee Police Department. “He was using that training,” Mr. Smith said.

The body camera footage was expected to be shown to the jury on Wednesday.

The shooting, initially defended by police leaders, touched off two nights of looting, arson and rioting on the city’s north side and drew attention to long-simmering tensions between law enforcement and Milwaukee’s black residents. The trial, being conducted amid heightened courtroom security and with a sequestered, anonymous jury, is expected to focus on whether Mr. Heaggan-Brown acted unreasonably in continuing to shoot after Sylville Smith tossed his gun. If convicted of first-degree reckless homicide, Mr. Heaggan-Brown could face up to 60 years in prison.

The defense said that Mr. Heaggan-Brown, 25, and two other officers were working overtime that day in Milwaukee’s Sherman Park neighborhood when they stopped Sylville Smith for being parked more than 12 inches from the curb.

Prosecutors have said that Sylville Smith was carrying a handgun as he ran away from Mr. Heaggan-Brown, then slipped and was shot in the biceps while throwing his weapon over a fence.

Opening statements centered on a frame-by-frame description of the body camera video, with the prosecution saying that the footage suggested that Mr. Heaggan-Brown fired a second shot almost immediately after the first, when Mr. Smith was unarmed and crumpled on the ground.

Both the prosecution and the defense emphasized how quickly the incident escalated: The entire foot chase after Sylville Smith got out of his car lasted 12 seconds or less. And the time between the police officer’s first bullet struck Mr. Smith in the arm and the second shot was just 1.69 seconds.

Mr. Chisholm, the prosecutor, said law enforcement experts would testify on Wednesday that Mr. Heaggan-Brown should only have used lethal force if faced with death or great bodily injury.

“Was that second round reasonable?” he asked.

But Jonathan Smith, the officer’s lawyer, said an experienced police trainer would testify that officers were taught to use the “one plus rule” — meaning assume that if a suspect has one weapon, the suspect often has another.

“A gunfight doesn’t end until the threat is stopped,” Mr. Smith said.

While such claims from defense lawyers mirror those heard in other police shooting trials, Mr. Heaggan-Brown’s case is different.

Both the officer and the deceased are young black men who grew up on the same side of town.

Also, weeks after the shooting, Mr. Heaggan-Brown was fired from the Milwaukee Police Department and charged with unrelated felony sex crimes, including the alleged sexual assault of a man one night after Sylville Smith’s death. Those charges are still pending, and a separate trial is expected later this year.