A witness to the fatal police shooting of Alejandro “Alex” Nieto in a San Francisco park said Thursday that Nieto had his hands in his jacket pockets when officers opened fire, contradicting the testimony of the officers who said Nieto pointed a Taser at them that they mistook for a gun.

Antonio Theodore took the stand on the third day of a civil trial at the U.S. District Court in San Francisco. Nieto’s parents are suing the city for wrongful death, arguing that four officers used excessive force when they fired 59 shots at Nieto on March 21, 2014, at Bernal Heights Park.

The city says Nieto’s pointing of his Taser caused police to mistake the device for a pistol and shoot in self-defense. Nieto, a 27-year-old City College of San Francisco student with a history of erratic behavior, carried the Taser for his job as a security guard, and authorities said that when it was recovered its barb-like electrodes had been released.

Theodore said he went to the park around 6:30 p.m. to jog with his dog and passed Nieto near a bench on the western side. He kept jogging, and upon approaching the gate to the east, observed two police cars. He said he stopped to see what was going on from a trail above the access road — about 115 feet, he estimated, from where the shooting would take place.

He described how one of the patrol cars drove up the road, stopped and then continued toward Nieto, whom Theodore described as walking down the road in a “very calm and casual way.” The car pulled up close to Nieto before two officers got out and took positions behind their doors.

“One officer yelled ‘stop’ — the officer on the driver’s side,” Theodore testified. “After the ‘stop,’ he fired the first shot at the man in the red jacket. He then fired another shot. Then a third shot. Then the person fell on their knees, and after a second he got a fourth shot and fell on his face.”

Theodore said that he could see Nieto and the officers clearly, despite a minor astigmatism, and that Nieto kept his hands in his jacket pockets during the encounter. He said he hadn’t spoken with police after the incident because he was scared and didn’t trust them; he said even facing the officers in court made him nervous.

City attorneys, who did not have a chance to cross-examiner Theodore on Thursday, believe that his story is not credible and that he didn’t have a clear view. The officers who first confronted Nieto, Richard Schiff and Lt. Jason Sawyer, testified earlier that Nieto threatened them before pulling the Taser from his hip and pointing it, causing it to emit a red laser beam.

“It was tragic,” Schiff said. “But unfortunately, I was forced.”

Also taking the stand Thursday was Justin Fritz, whose 911 call about a suspicious person drew officers to the park. He said he didn’t see the shooting, but heard two volleys of gunfire with a pause in between and had to duck for cover as bullets ripped into trees near him.

Fritz, who had gone to the park to walk his dogs with his partner, said that when he initially passed Nieto, who was next to a bench and appeared to be eating, he didn’t notice anything unusual. Minutes later, though, his partner told him he’d seen a gun on Nieto’s hip and that Nieto appeared to be practicing drawing it.

Fritz called 911 and was torn between the dispatcher’s request that he remain and watch Nieto and his partner’s pleas that they flee the park. “All I could keep playing in my head was an active shooter situation like Columbine,” Fritz said.

Retreating toward stairs that led to the street, Fritz kept an eye on Nieto while trying to conceal himself. At that point, he said, Nieto didn’t appear to be making any wild gestures or talking to himself.

But Fritz lost sight of Nieto, and roughly 45 seconds later, shots rang out — first a volley of what sounded like eight or nine blasts, he said, then a 10-second pause and another volley. Still on the phone with the dispatcher, Fritz took off down the stairs with his partner.

“Oh s—, there’s gunfire! There’s gunfire!” Fritz can be heard saying on a recording played for the jury. “They’re firing at me! Help! Help! Help!”

The account of a pause in gunfire, also described in Theodore’s testimony, is likely to be a key area of dispute. Adante Pointer, an attorney for Nieto’s parents, argues that it was unreasonable that officers would empty their guns and even reload when faced with a subject who did not fire back.

But city attorneys — and prosecutors who cleared the officers of charges — say police faced a threat that appeared to be life-threatening. Two later-arriving officers shot at Nieto after he went to the ground, saying his head and the stun gun were raised.

Earlier Thursday, Officer Nathan Chew, one of the later-arriving officers, testified that as he entered the park he heard six or seven shots. He said he drove to where Schiff and Sawyer were parked, with their guns trained toward Nieto, who was on the ground in what Chew described as a prone position with an object in his hand that appeared to be a gun that emitted a red laser beam.

Though he couldn’t recall hearing any gunfire coming from Nieto, Chew said he shot five times because he feared for his life and that of his fellow officers before he saw Nieto’s head drop.

Kale Williams is a San Francisco Chronicle staff writer. Email: kwilliams@sfchronicle.com Twitter: @sfkale