Rioters have plunged Baltimore into chaos, setting police cars ablaze and throwing bricks at officers hours after thousands mourned a man who died from a spinal injury he suffered in police custody.

The governor declared a state of emergency and called in the National Guard to restore order - but authorities were still struggling to quell pockets of unrest.

The riot was the latest flare-up over the mysterious death of Freddie Gray, whose fatal encounter with officers came amid the national debate over police use of force, especially when black suspects are involved.

Mr Gray was African-American. Police have declined to specify the races of the six officers involved in his arrest, all of whom have been suspended with pay while they are under investigation.

The violence began in West Baltimore, within a mile of where Mr Gray was arrested and pushed into a police van earlier this month.

Attorney General Loretta Lynch, in her first day on the job, said she would send Justice Department officials to the city in coming days.

A week-long, daily curfew was imposed beginning from 10pm to 5am, the mayor said, and Baltimore public schools announced they would be closed today.

At least 15 officers were hurt, including six who remained in hospital. Two dozen people were arrested.

Officers wearing helmets and wielding shields occasionally used pepper spray to keep the rioters back. For the most part, though, they relied on line formations to keep protesters at bay.

But Mr Gray's family said violence is not the way to honour him.

"I think the violence is wrong," his twin sister, Fredericka Gray, said. "I don't like it at all."

The family's lawyer, Billy Murphy, said they had hoped to organise a peace march later in the week.

Emergency officials were constantly thwarted as they tried to restore calm in the affected parts of the city of more than 620,000 people.

Firefighters trying to put out a blaze at a store were hindered by someone who sliced holes in a hose connected to a fire hydrant, spraying water all over the street and nearby buildings.