A US mayor has promised to thoroughly investigate the death of an African American man who died after an arrest that lawyers say left him with severe neck injuries.

The case in Baltimore is the latest in a long series of deaths that critics say demonstrates officers' racial bias and an over reliance on force when dealing with suspects.

Freddie Gray, 25, who died on Sunday, was arrested on April 12 and suffered an 80 per cent severed spinal cord and three broken vertebrae during the ordeal, his family's lawyer said.

Police have yet to announce how he was injured or why he was arrested.

"While in police custody for committing no crime - for which they had no justification for making the arrest except he was a black man running - his spine was virtually severed, 80 per cent severed, in the neck area," lawyer William Murphy Jr told a news conference, according to the Baltimore Sun.

About 100 local residents and activists gathered outside a local police station Sunday demanding more information.

Baltimore Mayor Stephanie Rawlings-Blake said on Monday officials were investigating the incident.

"We have to make sure that we are doing this investigation in a way that ensures that the people who need to be held accountable can be and will be held accountable," she told CNN.

There would also be an "independent review" of the investigation, she said.

A video shot from nearby showed police restraining Gray on a footpath, then dragging him yelling in pain to a police van.

Baltimore is the largest city in the state of Maryland, about 56 kilometres northeast of Washington DC.

A series of recent killings of unarmed black men by police officers has sparked nationwide protests, charges of racism and revived the debate about excessive use of police force.