Michael White made a series of unnecessary choices before he plunged a knife into Sean Schellenger’s back in July 2018 near Rittenhouse Square, prosecutors said Thursday.

He stopped his bicycle at 17th and Chancellor Streets, inserted himself into an argument that Schellenger was having with a motorist, then pulled out a weapon and stabbed Schellenger — whom he did not know — after the two men got into a scuffle, Assistant District Attorney Anthony Voci told jurors during opening arguments in White’s trial.

“An unarmed human being died as a result of a traffic dispute,” Voci said.

But one of White’s attorneys, public defender Jonathan Strange, told jurors that White was acting in self-defense, and that White will testify that Schellenger was drunk and told him, “I’m going to beat the black off you,” before charging and trying to take him to the ground.

“Michael White was scared for his life,” Strange said.

Neither side disputes that White killed Schellenger. And for the first time Thursday, cell-phone video of the killing was played in court. It shows Schellenger lifting White into the air in an attempt to tackle him, and White reaching over Schellenger’s shoulders and stabbing him in the back.

The two men then collapse onto the ground together, and White pulls the knife out of Schellenger’s back before running away on Chancellor Street.

The central question for jurors then will be whether they believe those actions should lead to a conviction for voluntary manslaughter — often referred to as a “heat of passion” killing or an unreasonable act of self-defense — or if White was justified and acted to protect himself.

The courtroom, one of the larger ones in the Stout Center for Criminal Justice, was packed Thursday, the first full day of testimony.

White, 22, sat quietly at a table next to his lawyers, as friends and supporters of both White and Schellenger filled the benches in the gallery.

Lawyers and spectators have been instructed not to comment until the trial is over.

The case has attracted widespread interest and stoked racial tensions almost since the fatal stabbing on July 12, 2018, and became the first high-profile test case of District Attorney Larry Krasner’s progressive approach to criminal justice.

Renewed controversy arose last week after Krasner asked to downgrade the charges against White from third-degree murder to voluntary manslaughter and obstruction-related counts.

That marked the second time Krasner has lowered the charges against White, who initially faced first-degree murder after his arrest. Krasner said last year that prosecutors did not have evidence to show that White intended to kill Schellenger, a 37-year-old real estate developer.

As for last week’s decision, Krasner said it was the “most likely way to secure a just conviction.” Schellenger’s relatives, meanwhile, criticized the DA for seeking to take power away from a jury tasked with determining the case’s outcome, as White already faced counts of third-degree murder and voluntary manslaughter.

Voluntary manslaughter carries a maximum prison sentence of 10 to 20 years. Third-degree murder is punishable by a 20-to-40-year sentence.

In addition to opening statements Thursday and the screening of the video, three eyewitnesses testified: Melanie Rice, who recorded the video; Erik Peterson, a host at the newly opened, nearby restaurant Spice Finch; and Norris Jordan, co-owner of the restaurants Happy Rooster and Lou Bird’s, who was driving Schellenger in his Mercedes-Benz immediately before the confrontation with White.

Jordan said he had left the restaurant Rouge with Schellenger and another man, Uri Jacobson, to drive to another restaurant. On the way, Jordan said, they got stopped in traffic on Chancellor, and Schellenger got out of the car and began talking to people in the cars stopped in front of them.

Jordan testified that he then heard an unknown voice say something to the effect of, “You want this? You don’t want this.” Jordan watched as Schellenger and White — whom Jordan did not know — confronted each other.

Peterson said he watched from inside the restaurant as Schellenger “charged” at White to initiate their scuffle. Peterson said he could not hear what the two men had said.

Rice’s video begins just as Schellenger appears to lift White into the air. The stabbing and their collapse last just seconds and seem like one continuous motion.

Schellenger flips over when he lands on the ground. White then pulls the knife from Schellenger’s back.

Jordan and Jacobson then can be heard on the video yelling at White, who runs west on Chancellor. Jacobson runs after him.

Schellenger lies motionless on the ground, blood staining his shirt, as Jordan and passersby await first responders.

Jordan had to pause at times during his testimony to wipe away tears. He was due back on the witness stand Friday morning.

The trial is expected to last through at least early next week.