A lawyer for six Baltimore police officers says they "did nothing wrong", after criminal charges were announced against them in the case of Freddie Gray who died in police custody.

Lawyer Michael Davey said the officers "at all times acted reasonably and in accordance with their training".

Baltimore state prosecutor Marilyn Mosby earlier said the death of the 25-year-old black man was a homicide and his arrest had been illegal.

Gray's death sparked violent protests.

But after the charges were announced on Friday afternoon, celebrations broke out across Baltimore. Drivers honked their car horns as people took to the streets with fists raised in triumph.

The spontaneous street celebrations turned into a small protest march in the evening, calling for an amnesty for those detained after Monday's rioting.

"These charges are an important step in getting justice for Freddie," said Gray's stepfather Richard Shipley.

All six officers have been suspended and were charged after they voluntarily presented themselves at the city jail. They have now been released from detention on bail.

'GRAVE CONCERNS'

Speaking to reporters on Friday, Davey accused Mosby of an "egregious rush to judgement".

"As all of the facts surrounding this case come out in the appropriate form, the officers' lack of wrongdoing will be made abundantly clear."

Davey insisted that Gray's injuries "did not occur as a result of any action or inaction on the part of these officers."

He also said that the defence team had "grave concerns about the fairness and integrity of the prosecution of our officers".

Mosby earlier rejected a police union request to step aside and appoint a special prosecutor to handle the case.

The prosecutor said the findings of an independent investigation, coupled with the medical examiner's view that the death was a homicide, had "led us to believe that we have probable cause to file criminal charges".

Mosby said Gray died as a result of injuries suffered while he was shackled inside a Baltimore police van, but not restrained by a seat belt - as he was legally required to be.

She said the officers failed to provide medical aid to Gray after he repeatedly pleaded for help.

The driver of the van, Caesar Goodson, 45, faces the most serious charge, second-degree murder. Goodson faces more than 30 years in prison if convicted.

The other officers face charges including involuntary manslaughter, assault and misconduct.

Mosby said that Gray was not carrying an illegal switchblade as reported earlier by police, but a legal pocketknife.

After Gray's funeral on Monday, riots broke out in sections of West Baltimore. About 200 people were arrested as more than 100 cars were set alight and 15 buildings destroyed.

Since then, the city and state officials deployed thousands of extra law enforcement officers and National Guard troops to keep the peace and enacted a citywide curfew.

Gray's death is the latest in a string of high-profile cases where black men have died after contact with the police.