ST LOUIS (REUTERS) - Hundreds of protesters poured into St Louis' streets and some scuffles broke out as they voiced their anger after a Missouri judge on Friday (Sept 15) ruled a white former St Louis police officer was not guilty of murder in the 2011 shooting of a black man.

With the National Guard on standby in case of violence, authorities appealed to protesters to march peacefully in a state where racially charged clashes in the nearby city of Ferguson spawned the Black Lives Matter movement in 2014.

Police said a few protesters threw rocks and water bottles, while witnesses and video showed officers used pepper spray on at least five people about a block from the courthouse. About 50 riot police were on hand, blocking a ramp to a nearby highway.

Jason Stockley, 36, was acquitted of first-degree murder for killing Anthony Lamar Smith, 24. The former policeman, who was arrested in May 2016, was accused of planting a gun in Smith's car but testified he acted in self-defence.

After the verdict, more than 500 protesters marched in downtown St Louis, chanting "No justice, no peace" and "Shut it down."

Some held "Black Lives Matter" signs.

"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."

Mayor Lyda Krewson and Governor Eric Greitens, who put the National Guard on standby, appealed for calm. Local schools planned early dismissals and businesses shut down.

"Frustration, anger, hurt, pain, hope and love all intermingle," Krewson said in a statement. "I encourage St Louisans to show each other compassion."

Al Watkins, an attorney for Smith's fiancee, Christina Wilson, said his client was appalled by the decision. He said the ruling showed prejudice, pointing to a line where the judge wrote that an "urban heroin dealer" without a weapon would be an anomaly.

Judge Timothy Wilson's highly anticipated ruling was announced more than five weeks after the bench trial ended.

"This court, as a trier of fact, is simply not firmly convinced of defendant's guilt," he wrote in his ruling.

Wilson said prosecutors also asked the court to consider a lesser degree of homicide but they did not prove beyond a reasonable doubt that Stockley's use of deadly force was not justified in self-defence.

Stockley's attorney, Neil Bruntrager, said the ex-officer on Friday was relieved and would seek to rebuild his life.

"It's been a long road for him," Bruntrager said.

'THIS AIN'T RIGHT'

In recent years, grand juries have declined to charge officers involved in the 2014 fatal shooting of Michael Brown, 18, which set of nights of violent protest in Ferguson, and the choking death of Eric Garner, 43, in New York.

Baltimore police officers also were not convicted in the case of Freddie Gray, who died from a broken neck suffered in a police van in 2015.

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 Fox 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."

Protesters chanted as they marched around downtown, at one point reaching police headquarters before moving on.

"What do you say to people here when they have no faith in the system and these buildings," said Kevin Nevels, 50, of St Louis, pointing to the courthouse.

Prosecutor Kimberly Gardner said she was disappointed with the verdict 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. "I am frustrated as well. Destruction of our community is not the answer."

Smith tried to flee from Stockley 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, the driver of the police car slammed into Smith's vehicle and they came to a stop, court documents said. Stockley then approached Smith's car and shot him five times with his service weapon.

Stockley's lawyers said he fired in self-defence, believing Smith was reaching for a gun. But prosecutors said the only gun recovered from the scene had only Stockley's DNA on it.

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 US$900,000 (S$1.2 million) in 2013, Watkins said.