Protests have taken place over the death of an African-American man who suffered severe neck injuries during his arrest.

Freddie Gray (25) died on Sunday, one week after he was taken to hospital with spinal injuries following an encounter with four police officers in Baltimore, Maryland.

The case follows a string of recent police killings of black men that critics say highlights officers' racial bias and an over-reliance on force when dealing with suspects.

Hundreds of residents and activists gathered outside the Baltimore police station where Mr Gray was taken, demanding questions about why he died.

Some held signs saying "Black Lives Matter" and "Unarmed!! One Man".

Mr Gray was stopped on April 12th and somehow suffered a broken vertebra during his arrest. Baltimore Police Department said he was running away from bicycle officers when he was arrested and put in a police van.

Amateur video shows him being dragged towards the van, but does not show the whole encounter.

Some 30 minutes later, he was taken to University of Maryland R Adams Cowley Shock Trauma Center in a critical condition.

"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% severed, in the neck area," said William Murphy Jr, an attorney for Mr Gray's family.

Baltimore Mayor Stephanie Rawlings-Blake said officials are investigating the incident.

"I want citizens to know exactly how it happened and if necessary, I will ensure we will hold the right people accountable," she told a news conference.

Deputy Commissioner Jerry Rodriguez added: "It's a two-part investigation. One is a criminal case, for Mr Gray and also the officers. We always have that component in there to determine whether there is criminal culpability."

Mr Rodriguez declined to specify why four officers stopped Mr Gray.

"We had officers in a high-crime area known to have high narcotic incidents," he said.

"The officers believe that Mr Gray was immediately involved or recently involved in criminal activity and decided to make contact."

The officers have not been identified, and are on administrative leave, as is routine during investigations into use of force.