BALTIMORE » In the clearest acknowledgement of failure yet, Baltimore police said Friday that Freddie Gray 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.

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 that led to his death. Authorities have not explained how or when it occurred. Six officers have been suspended with pay.

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.

Gray, who is black, 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 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."

Protesters promised their biggest march Saturday, when they would try to "shut down" the city.

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 thanked protesters for being peaceful so far and urged calm.

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

She expects the results of the investigation to be turned over to prosecutors in a week, and they will decide whether any criminal charges will be filed. There is no timetable for when that will. Police said prosecutors will likely ask them to do more investigating.

Asked if Gray’s possible "rough ride" is a one-off, the mayor 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," she said, referring to Dondi Johnson. He 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 leader of a group of local ministers called on Batts to resign immediately.

"It seems that no one in the police department can explain what happened," said the Rev. Alvin Gwynn Sr., president of the Interdenominational Ministerial Alliance of Baltimore.

He said the police department is "in disarray" and Batts has shown a "lack of viable leadership capabilities."

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.

The Rev. Frank Reid of Bethel African Methodist Episcopal Church in Baltimore, who met with Rawlings-Blake on Friday, said he wants to believe the police department’s investigation will be transparent.

"I want to take them at their word but the purpose of being here today is to not only to let the city know but the police department know that we’re going to hold them accountable," Reid said. "This is not going to go away … Business as usual is no longer acceptable. This is all too frequent. It’s a historic issue and we want to stop it now."