Washington, United States: Protests broke out in Baltimore and spread to Philadelphia on Thursday over the death of an African-American man, as fresh claims emerged of how Freddie Gray sustained his fatal injuries in police custody.

Baltimore has taken center stage in just the latest bout of protests in the United States over alleged police brutality and racism, echoing demonstrations that erupted in a St Louis suburb last year when a white policeman shot dead an unarmed black teenager.

Demonstrations flared in major US cities when a grand jury declined to indict the officer and there was a repeat on Wednesday when thousands again hit the streets in Baltimore, New York -- where police made 143 arrests -- the capital Washington and Boston.

Philadelphia on Thursday became the next US East Coast city to see protests demanding justice for Gray and an end to what demonstrators say is overly aggressive policing, particularly in confronting black Americans.

About 600 people gathered in Philadelphia, police said, and a similar number marched for a successive day in Baltimore, a city of 620,000 an hour's drive from Washington that has witnessed some of its worst unrest in decades.

But there was no immediate return to the scenes that made worldwide headlines on Monday when violence and looting shook Baltimore following Gray's funeral.

Gray died with 80 percent of his spine severed at the neck, lawyers for his family say, portraying him as just the latest young African American to die at the hands of the police in the United States.

New claims emerge

The circumstances surrounding Gray's April 12 arrest and how he sustained the injuries that killed him a week later remain murky and the subject of intense speculation.

Adding to the simmering anger on the streets, WJLA, an ABC affiliate, cited ‘multiple law enforcement’ sources as saying that a medical examiner found that the spinal injury was caused when he slammed into the back of the police transport van after his arrest, breaking his neck.

A head injury he suffered matches a bolt in the back of the van, the report said, stressing that it was not immediately clear what propelled him into slamming into the back of the vehicle.

Adding to the intrigue, police revealed that the van made a previously undisclosed stop between when Gray was arrested and when the vehicle arrived at the police precinct.

Baltimore Deputy Police Commissioner Kevin Davis did not elaborate on the significance of this.

Police and Gray's family have repeatedly appealed for calm and an emergency nighttime curfew, starting at 10:00 pm and running till 5:00 am, remains in place.

Since Monday's riots, a total of 98 Baltimore police officers have been injured, of whom 43 required emergency medical treatment.

Detectives probing Gray's death have handed their investigation over to prosecutors, but Baltimore Police Commissioner Anthony Batts urged people not to rush to conclusions.

‘I understand the frustration, I understand the sense of urgency, and so has the organization and that is why we have finished (the probe) a day ahead of time,’ Batts said.