Hundreds of protesters poured into St. Louis streets on Friday, and some briefly scuffled with police, after a Missouri judge acquitted a former white police officer of murder in the 2011 fatal shooting of a black man suspected of dealing drugs.

With National Guard troops placed on standby, authorities urged demonstrators to remain peaceful as they protested the outcome of a case reminiscent of circumstances that spawned racially charged unrest in the nearby suburb of Ferguson and gave rise to the Black Lives Matter movement in 2014.

Police said several rocks and water bottles were thrown, and officers were seen in video footage and by eyewitnesses dousing at least five people with pepper spray near the courthouse.

But the protests, while boisterous, were otherwise largely peaceful, and no serious injuries or acts of vandalism were reported.

As night fell, police reported making 13 arrests and said four officers had been assaulted, though the confrontations were described as fairly minor. One group of demonstrators tried to climb onto Interstate 40 but was blocked by police officers in riot gear. Another group blocked an intersection by sitting down in the street for six minutes of silence.

After most protesters drifted away late on Thursday, a smaller group of individuals police described as "agitators" lingered on the streets in a neighborhood near the mayor's home, taunting officers who arrived in force.

Police fired volleys of tear gas as they ordered the crowd to disperse.

The outcry was prompted by Friday's verdict, rendered five weeks after the conclusion of a non-jury trial, finding former city policeman Jason Stockley, 36, not guilty of first-degree murder in the shooting death of Anthony Lamar Smith, 24.

Smith was shot five times in his car after trying to flee Stockley and his partner on Dec. 20, 2011, following an alleged drug deal, authorities said.

During the pursuit, Stockley could be heard saying on an internal police car video he was going to kill Smith, prosecutors said.

At Stockley's direction, his partner, who was driving, slammed the police cruiser into Smith's vehicle and they came to a stop, court documents said. Stockley then approached Smith's car and opened fire with his service weapon.

The former policeman believed Smith was armed, defense attorneys said, and a gun was found in the car. But prosecutors argued Stockley planted the weapon and that the gun had only Stockley's DNA on it.

Following the verdict, some 600 protesters marched through downtown St. Louis, chanting "No justice, no peace" and "Hey hey! Ho ho! These killer cops have got to go!" Some held signs that read "Black Lives Matter" and "No more racist killer cops."

Later in the evening, several hundred protesters moved from the courthouse to the city's popular restaurant neighborhood, the Central West End, and the crowd began to grow again.

"I’m sad, I’m hurt, I’m mad,” the Reverend Clinton Stancil of the Wayman AME Church in St. Louis said by telephone. “We haven’t made any progress since Ferguson, that’s clear. Cops can still kill us with impunity."

Stockley's attorney, Neil Bruntrager, said his client was relieved at the verdict. “It’s been a long road for him,” Bruntrager said.

In his ruling, Judge Timothy Wilson said he doubted the prosecution's claim the gun was planted, writing: "the court observes, based on its nearly 30 years on the bench, that an urban heroin dealer not in possession of a firearm would be an anomaly."

The comment sparked outrage by protesters on the street and on social media.

Murder convictions against law enforcement officers are rare. In recent years grand juries have declined to even charge officers involved in the 2014 fatal shooting of Michael Brown, 18, which triggered waves of violent protests in Ferguson, and in the choking death of Eric Garner, 43, in New York. Both were unarmed and black.

Brown's father, Michael Brown Sr., voiced his frustration after Friday's verdict.

"You all know this ain't right and you all continue to do this to us," he told a St. Louis television station. "Like we don't mean nothing, like we're rats, trash, dogs in the streets. Right now, I'm praying for my city because my people are tired of this."

St. Louis prosecutor Kimberly Gardner said she was disappointed and called on protesters to avoid violence.

“I understand the verdict has created anger and frustration for many in our community," she told reporters at the courthouse.

Stockley waived his right to a jury trial, allowing the judge to decide. He left the St. Louis Metropolitan Police Department in 2013 and was arrested last year.

Smith's family settled a lawsuit against the city for $900,000 in 2013, according to Al Watkins, an attorney for Smith's fiancee, Christina Wilson.