Thousands of protesters have hit the streets of Baltimore again despite the charging of six police officers over the death of a 25-year-old African American whose spine was snapped in custody.

The charges - ranging from second-degree murder and manslaughter to misconduct - were set out on Friday in a surprise announcement by Maryland state prosecutor Marilyn Mosby.

All six officers - three of them black and three white, according to mug-shots broadcast by CNN - were taken into custody and later posted bond, reports said.

The death of Freddie Gray, 25, just the latest black American to lose his life at the hands of police, has reignited simmering resentment in the United States over police tactics, particularly in their dealings with African Americans.

"The findings of our comprehensive, thorough and independent investigation, coupled with the medical examiner's determination that Mr Gray's death was a homicide... have led us to believe that we have probable cause to file criminal charges," Mosby said.

Mosby said Gray "suffered a severe and critical neck injury as a result of being handcuffed, shackled by his feet and unrestrained" inside a moving police van following his arrest on April 12.

En route to a police station, the police van stopped at least three times, including once to pick up a suspect in an unrelated case.

Cheers broke out when Mosby unveiled the charges on the steps of Baltimore's war memorial, across the street from city hall, a focal point of protests demanding justice and change.

Baltimore's police union condemned what it called "an egregious rush to judgment" as it defended the officers and expressed confidence they would be vindicated.

But if nervous authorities had hoped the announcement would ease palpable tensions on the streets, a march that grew bigger by every passing hour suggested otherwise.

Several thousand people rallied from City Hall through downtown streets lined with riot police, demanding justice and an end to alleged racism and police brutality.

Some held aloft placards thanking Mosby, but others cautioned that the charges were only a first step.

William Murphy, a lawyer for the Gray family, told reporters they had no advance word of the charges.

Gray, who had a record of non-violent drug offences, died a week after his arrest from spinal injuries sustained when he was arrested in a west Baltimore public housing project.

Gray has become the latest face of an intense national debate over whether American police are too quick to use violence against unarmed black males.

In the best-known case, a white police officer fatally shot unarmed teenager Michael Brown in Ferguson, Missouri - prompting riots in the St. Louis suburb - but was not indicted by a grand jury.

At the White House, President Barack Obama said it was "absolutely vital" for the truth in Baltimore to come out.