Washington, Sep 16 (IANS): A US judge found former police officer Jason Stockley not guilty of first-degree murder of an African-American man, the media reported.

After the verdict on Friday, protesters and activists gathered outside the courthouse in St. Louis, Missouri, and marched through the city for hours, reports CNN.

Some could be seen locking arms and praying together, while others held signs, chanting, "No justice, no peace".

While for the most part the protests started peacefully, St. Louis police said on Friday evening that demonstrators threw rocks at the mayor's home -- breaking windows and smashing the windshield of a police vehicle.

They also hit officers with water bottles and other items leading to the arrest of at least 13 protesters.

"I will protect people's constitutional right to peacefully protest, but violence will not be tolerated. We will protect people's lives, homes, and communities," Missouri Governor Eric Greitens said.

In December 2011, Stockley, then a St. Louis officer, fatally shot Anthony Lamar Smith, 24, after a police chase over a suspected drug deal, CNN reported.

After he pleaded not guilty to a murder charge, Stockley waived his right to a jury trial, meaning the ruling was left to the judge.

Stockley had said he acted in self-defence and believed Smith was reaching for a gun in his car, but prosecutors accused the officer of planting a silver revolver to justify the shooting.

In Friday's ruling, St. Louis Circuit Judge Timothy Wilson said the state failed to prove that Stockley did not act in self-defence.