Thousands of protesters have taken to the streets to demand answers over the police custody death in the US.

The demonstration in Baltimore, Maryland, was peaceful for hours, but later pockets of protesters smashed police car windows and shopfronts.

The protests over the death of 25-year-old black man Freddie Gray came a day after deputy police commissioner Kevin Davis said Mr Gray should have received medical attention at the spot where he was arrested, before he was put inside a police van handcuffed and without a seat belt - against force policy.

Mr Gray died on April 19 after suffering a fatal spinal injury while in custody and his death has intensified a national debate over police treatment of African-Americans.

Authorities have not explained how or when Mr Gray's spine was injured. Video showed him being dragged into a police van and police have said he was in it for about 30 minutes before paramedics were called.

Mr Gray's death has been compared to those of other unarmed black men who died at the hands of police in New York City and Ferguson, Missouri.

Baltimore residents voiced their anger at how the police and the city are handling the investigation into Mr Gray's death.

Protesters threw cans and plastic bottles in the direction of police officers. One protester broke the window of a police car, grabbed a police hat inside and wore it while standing on top of the vehicle with several others.

At least two people were hurt in the mayhem and at least a dozen were arrested.

In her first public comments since Mr Gray's death, his twin sister Fredricka Gray appealed for calm.

"My family wants to say, can you all please, please stop the violence?" she said at a news conference with the mayor. "Freddie Gray would not want this."

Earlier the crowd paused for a moment of silence in front of Shock Trauma, the hospital where Mr Gray died.

Carrying slogans such as "Jail Killer Police!" and "Unite Here!", demonstrators filled two city blocks and marched to City Hall, where the crowd filled a grassy plaza.