The police used tear gas to disperse the demonstrators, opening fire on looters and injuring around 20 people, many of whom sustained gunshot wounds, according to hospital officials contacted by AFP.
Large crowds gathered in the streets and hundreds of youths ransacked and set fire to a police station, killing an officer, national police spokesman Mamadou-Alpha Barry told AFP.
"Sacks of rice, computers, a generator, weapons, police uniforms et cetera were taken by protesters who chanted 'down with the police, down with law enforcement, down with criminals, down with the killers'," another witness told AFP.
A police patrol had arrested the youth, a well-known local footballer named Balla Conde, in the eastern city of Kankan on Saturday night before repeatedly beating him and leaving him to die in the street, several witnesses told AFP.
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"He was picked up by his family and driven to the central city hospital where he died on Sunday and he was buried on Monday morning," one witness said.
Regional governor Nawa Damey told AFP that Conde was famous in Kankan, Guinea's largest city by area with a population of more than 200,000.
It is not known why the youth was detained. But Damey said the chief of police had launched an investigation and a number of suspects had been arrested over the killing.