DeRay Mckesson is one of the best known voices of the Black Lives Matter movement and he was arrested July 9 while protesting recent police shootings of black men in Baton Rouge and suburban St. Paul. Here's what else you should know about him. (Monica Akhtar,Whitney Shefte/The Washington Post)

DeRay Mckesson, one of the most prominent activists associated with the police reform protest movement, was arrested late Saturday in Baton Rouge, where he traveled to demonstrate in solidarity with residents angered by the recent death of Alton Sterling after an officer-involved shooting that was captured on video.

He was released Sunday afternoon after being taken into custody about 11 the night before.

Two fellow activists who witnessed the arrest described it as physically violent.

“The officers won’t give their names,” said Brittany Packnett, a co-founder with Mckesson of the group Campaign Zero, a prominent activist collective. “He was clearly targeted.”

She later tweeted that 100 people were arrested in Baton Rouge. A Baton Rouge parish prison official later told The Washington Post that more than 120 people were arrested across multiple protest sites.

Mckesson was charged with obstructing a highway of commerce, said the official, who spoke on the condition of anonymity because she was not authorized to talk to reporters.

“He was held an inordinate amount of time,” said Roy J. Rodney Jr., Mckesson’s attorney.  “And normally people who peacefully protest are not arrested in this fashion.”

A probable cause affidavit, obtained by Yamiche Alcindor of the New York Times, states that authorities allege that Mckesson twice went onto a road before he was taken into custody.

“During the protest, the defendant entered the roadway and was provided another verbal order to exit the lanes of travel,” the affidavit states. “Moments later, the defendant entered the roadway again and was taken into custody by officers on the scene without incident.”

Protesters had been told to stay out of the road and not disrupt traffic, according to the affidavit.

“I was with local activists when I was arrested yesterday,” Mckesson told The Post after his release. “I was in compliance with the law, and I am confident that this was an unlawful arrest.”

Packnett said Mckesson was using his smartphone to live-stream the ongoing protests when police began forcibly dispersing the crowds. As Mckesson and a group of about eight people walked down the street, an officer approached him and told him that he had been “flagged” and that he would be arrested if he left the sidewalk again.

Moments later, she said, two officers forcefully arrested Mckesson.

“They tackled him. One officer hit the top of his body and another officer, the bottom,” Packnett said.

The altercation knocked the phone from Mckesson’s hand, ending his live broadcast of the demonstration, she said.

Blurry video of the moments before he was taken into custody that was provided to The Post captures his verbal exchange with the officers.

“The police continue to just provoke people,” Mckesson says after an officer yells to a group of people that if they step on the roadway, they will be arrested.

Then an officer says the man in the “loud shoes” has been “flagged”: “You in them loud shoes, if I see you in the road, if I get close to you, you’re going to jail,” an officer can be heard saying on the video.

In response, Packnett says: “We’re on the shoulder. There is no sidewalk, sir.”

Mckesson is known for wearing a pair of red Nike sneakers and a blue vest to the protests he attends.

The group was walking away from a protest that had been dispersed, traveling alongside road traffic on a street that they said does not have a sidewalk.

Activists continued to talk as they walked up the side of the street. Moments later, an officer’s voice is heard: “City police, you’re under arrest.”

“What?” Mckesson exclaims. “I’m under arrest, y’all.”

Then the video and audio feed ends.

As Packnett and Johnetta Elzie, another prominent protester, tweeted in outrage, word spread quickly through the ranks of national police-reform activists, who feared that Mckesson was targeted deliberately and may be harmed in police custody.

In a text message from police custody, Mckesson had said he and 33 others were in custody together, wrists tied, and being taken to a police precinct.

News of Mckesson’s arrest exploded on social media, with more than 100,000 tweets before dawn using the hashtag #FreeDeRay. Many had urged people to call Baton Rouge police to demand his release.

Sarah Larimer contributed to this report, which has been updated.

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People in Dallas, St. Paul, Minn., and across the nation grapple with how to move on from the deaths of two black men at the hands of police as well as the loss of five police officers all in one week's time. (Whitney Shefte,McKenna Ewen,Dalton Bennett,Ashleigh Joplin/The Washington Post)