Protests have turned violent in the US city of St. Louis after police confronted demonstrators who were out in the streets against the acquittal of a white former police officer in the shooting death of black man several years ago.

Protests, mostly peaceful, began early Friday when Judge Timothy Wilson cleared former police officer Jason Stockley in the 2011 murder of black driver Anthony Lamar Smith.

Protest continued throughout the day and into the night with police using tear gas against people and arresting at least 30 protesters, interim St. Louis Police Chief Lawrence O�Toole said early Saturday. He said some nine police officers were injured.

“Many of the demonstrators were peaceful," O’Toole said. “However, after dark many agitators began to destroy property and assault police officers."

Police said that "agitators have converged on Mayor Krewson�s house. Throwing rocks and breaking windows, despite being instructed not to."

Police said they had to use tear gas and pepper balls on the crowd after some officers were hurt by thrown bricks.

Back on December 20, 2011, 36-year-old Jason Stockley shot 24-year-old Lamar Smith five times after a high-speed chase and crash.

Stockley had said he acted in self-defense and believed Smith was reaching for a gun in his car. Prosecutors, however, accused the officer of planting a gun in Smith�s car.

In the recent ruling, Judge Wilson said the state failed to prove that the officer did not act in self-defense.

"This Court, in conscience, cannot say that the State has proven every element of murder beyond a reasonable doubt, or that the State has proven beyond a reasonable doubt that the defendant did not act in self-defense,” the judge wrote in his ruling.

St. Louis Circuit Attorney Kimberly Gardner, however, said she was “disappointed” with the decision. 

Protesters said they would meet again Saturday to plan further demonstrations.

The St. Louis region has formerly been gripped by violent protests back in 2014, when white police officer Darren Wilson fatally shot unarmed teenager Michael Brown.

Since then, police shootings and following decisions not to charge the officers in most of the cases involved, have sparked protests across the US including, New York, Chicago, Los Angeles, Baltimore, San Francisco, and many other cities.