A Cleveland police officer was found not guilty on Saturday in the shooting deaths of two unarmed black people after a high-speed car chase in 2012, one in a series of cases that have raised questions over police conduct and race relations in the United States.

Judge John O'Donnell said Officer Michael Brelo, 31, acted reasonably in shooting the two suspects, a man and a woman, through the windshield. Brelo was found not guilty of two charges of voluntary manslaughter and one charge of aggravated assault.

While most US criminal defendants choose to have their cases judged by a jury of their peers, Brelo had requested that a judge hear his case.

Brelo, 31, was one of 13 officers who opened fire on Timothy Russell and Malissa Williams during a police chase on November 29, 2012.

The couple's car had backfired as it drove past Cleveland police headquarters, and police thought the sound was a gunshot.

A total of 137 rounds were fired at the car, including 49 by Brelo.

He shot the final 15 from the hood of Russell's Chevrolet Malibu.

Protesters took to Cleveland streets on Saturday night as police patrolled in riot gear. Cleveland police spokeswoman Jennifer Ciaccia said that over 20 people were arrested.

Brelo's trial, which began on April 6, took place at a time when US law enforcement is under scrutiny for the use of lethal force against minority groups. It followed a series of high-profile deaths of unarmed black men in confrontations with police, which have prompted sometimes violent demonstrations.

Soon after the verdict, a small crowd of demonstrators took to the streets chanting "No justice, no peace," with protests becoming larger and more unruly. Police arrested at least three people at a restaurant after someone threw an object through a window and injured a customer.

Reaction to the verdict was swift on social media, with many saying they were bewildered. US Representative Marcia Fudge, a Democrat from Ohio, called the decision a "stunning setback on the road to justice".

Reuters - AFP