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Protesters in Baltimore have begun a week of demonstrations after the death of a man in police custody.

Freddie Gray, 25, died on Sunday after suffering a fatal spine injury under unexplained circumstances after his arrest earlier this month.

The US Justice Department has opened a civil rights investigation.

A thousand people gathered on Tuesday at the site of his arrest, kicking off seven days of protests against what they believe is excessive police force.

As people raised their hands, Pastor Jamal Bryant, one of the demonstrations organisers, said the gesture was not an act of surrender.

"It's a sign of strength, of one unity and one commitment that we will not rest until we get justice for Freddie Gray," he said.

Image copyright Getty Images

Image copyright Getty Images

Gray was arrested on 12 April. Officials said that he ran away after he "made eye contact" with officers on patrol.

Officers pursued him and took him into custody minutes later. The officers' reasoning for detaining Gray is not clear.

"There is no law against running," Police Commissioner Anthony Batts told reporters on Monday.

"Running while black is not probable cause," Billy Murphy, a lawyer hired by the Gray family, said.

Police timeline of the arrest

Image copyright Murphy, Falcon & Murphy

Sunday, 12 April, 0839: Officers approach Gray and he flees on foot

Officers approach Gray and he flees on foot 0840: Gray arrested on corner of Presbury Street, Sandtown

Gray arrested on corner of Presbury Street, Sandtown 0842: Police request a van

Police request a van 0854: Van departs with Gray inside, conscious and speaking

Van departs with Gray inside, conscious and speaking 0924: Police request paramedics to take Gray to hospital

What we know about Gray's death

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Image copyright AP

Mobile phone video shows police pinning Gray to the ground before a detention van arrives to transport him to a police station.

In the video, officers are seen dragging Gray's limp body into the van. It is not known whether Gray's body was limp because of a deliberate act of defiance or because of a medical condition.

Gray was in the van for approximately 30 minutes. At one point, police stopped to put Gray into leg shackles after determining he had become irate.

He asked officers for medical care on multiple occasions, Mr Batts said.

Image copyright Getty Images

Image copyright Getty Images

He was eventually rushed to hospital where he lapsed into a coma and died a week later.

A national debate over the use of police force has been going on since last summer, when a black teenager was killed in Ferguson, Missouri.

"The world is watching, and the world needs to see that black Baltimore is unified," said Pastor Bryant.