Baltimore police, in the clearest acknowledgement of failure yet, said Friday that a black man who later died should have received medical attention at the spot where he was arrested — before he was put inside a police van.

Officers missed "multiple" opportunities to give him medical attention, and once inside the van, Gray should've been buckled into a seat belt.

The department's acknowledgement came at a news conference after a week of intense scrutiny and near-daily demonstrations over what protesters say is police mistreatment of blacks in Baltimore and throughout the country. A fierce national debate has been stoked by the deaths of unarmed black men in Ferguson, Mo., and New York City.

Gray was taken into custody April 12 and at some point — either during his arrest or inside the van — he suffered a mysterious spinal injury. Authorities have not explained how or when it occurred. Six officers have been suspended with pay.

Not buckled in

Commissioner Anthony Batts said it was possible Gray was injured before the van ride, but also possible he suffered in a "rough ride" — where officers hit the brakes and take sharp turns to injure suspects in the back of vans.

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Organizers are expecting thousands of people to show up for further protests this Saturday. (Patrick Semansky/Associated Press)

Gray was arrested after he made eye contact with officers and ran away, police said. Officers held him down, handcuffed him and loaded him into a police van. While inside, he became irate and leg cuffs were put on him, police have said.

Gray was not buckled in, a violation of the police department's policy.

Gray asked for medical help several times, and after a 30-minute ride that included three stops, paramedics were called.

We know our police employees failed to give him medical attention in a timely manner multiple times. - Anthony Batts, Baltimore Police commissioner

"We know he was not buckled in the transportation wagon as he should have been. There's no excuse for that, period," Batts said. "We know our police employees failed to give him medical attention in a timely manner multiple times."

Deputy police commissioner Kevin Davis said Friday that Gray should have received medical attention at the spot of his arrest. Bystander video shows Gray screaming as officers carried him to the van, his legs appearing limp.

Batts said the investigation is being refined and the picture is getting "sharper and sharper." He did not elaborate.

As for some calls for his resignation, he said: "That's not going to happen."

Protests planned for Saturday

People angry over Gray's unexplained police-custody death promised their biggest march Saturday, when they would try to "shut down" the city.

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A Baltimore police captain speaks with a protester during one of the many marches that have taken place in the past week expressing outrage at Gray's death. (Patrick Semansky/Associated Press)

The president of a black lawyers' group predicted thousands of people would turn out, when good weather is forecast and the Baltimore Orioles host the Boston Red Sox.

"Things will change on Saturday, and the struggle will be amplified," said Malik Shabazz of Black Lawyers for Justice.

Shabazz rejected the notion that he was an outside agitator who would stir up trouble.

The mayor on Friday thanked protesters for being peaceful so far and said the police commissioner assured her the investigation into Gray's death is moving as quickly as possible. She expects the results to be turned over to prosecutors in a week. They will decide whether any criminal charges will be filed.

"I will not deny we have had a very long and complicated history on issues such as these," Mayor Stephanie Rawlings-Blake said Friday. "But it's important to remember that we have an equally long history of peaceful and legal protest."

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Baltimore Mayor Stephanie Rawlings-Blake supported the police commissioner but acknowledged that the city has 'a very long and complicated history on issues such as these.' (Patrick Semansky/Associated Press)

Asked if Gray's possible "rough ride" is a one-off, she said: "It's clearly not a one-off. The reason we have the policy around seat belts in the police vans is because of an incident that happened previously."

Rawlings-Blake was referring to Dondi Johnson, who died of a fractured spine in 2005 after he was arrested for urinating in public and transported without a seat belt, with his hands cuffed behind his back.

The mayor appeared to back the police commissioner at her own news conference, and Batts defended his record, saying he was brought on in 2012 to reform the department. Since then, he has fired 50 employees and reduced the number of officer-involved shootings and excessive force complaints.