Police arrested a prominent Black Lives Matter activist and more than 200 other people late Saturday and early Sunday on a night of tension and unrest during protests over the recent fatal shootings of black men.

DeRay Mckesson, one of the most visible faces of the Black Lives Matter movement – and an alumnus of Bowdoin College in Brunswick, Maine – was among 120 people arrested in Baton Rouge, Louisiana, and at least 100 others were detained in St. Paul, Minnesota, following what police described as rioting there that left 21 officers injured.

Mckesson was freed Sunday afternoon, authorities reported. No details were immediately available on the conditions of his release.

The protests began Saturday with demands for accountability from police and stretched into early Sunday in Baton Rouge and St. Paul, where tensions are most raw after the deaths of Alton Sterling in the Louisiana city and of Philando Castile in a St. Paul suburb.

With activists and police on edge after those deaths and the sniper killings of five police officers in Dallas, the United States is “sitting on a powder keg,” said Charles H. Ramsey, a former police chief in Washington and Philadelphia.

President Obama plans to travel to Dallas on Tuesday at the invitation of the city’s mayor to deliver remarks at an interfaith memorial service for the slain officers, the White House announced Sunday.

In a news conference Sunday in Spain, where he cut short a European trip because of the unrest in the United States, Obama said, “Any violence directed at police officers is a reprehensible crime.” He said that while protests are protected by the long-standing American right to free speech, those who engage in overheated rhetoric against police are “doing a disservice to the cause.”

“I think that the overwhelming majority of people who are involved in the Black Lives Matter movement, what they really want to see is a better relationship between police and the community,” Obama said. At the same time, he added, “I would hope that police organizations are also respectful of the frustrations people in these communities feel and not just dismiss these protests and complaints.”

A Baton Rouge parish prison official told The Washington Post on Sunday morning that about 120 people were arrested overnight at multiple protest sites across the Louisiana capital.

Mckesson was charged with obstructing a highway of commerce, said the prison official, who spoke on the condition of anonymity because she was not authorized to talk to reporters. She said bond had not been set for the arrested protesters as of late Sunday morning. Another official said bonds for demonstrators arrested the previous night were set between $250 and $400.

A late night sit-in on a Minnesota highway broke up after smoke bombs and flash-bang grenades were used by officers, who were the targets of rocks and water bottles thrown by protesters. During the confrontation, five officers suffered minor injuries – including one struck in the head with a large piece of concrete, a St. Paul Police Department spokesman tweeted. He later told the Star Tribune newspaper that about 50 people were arrested during that incident and 50 more later in the night.

Police subsequently said that 21 officers were injured.

“We will not tolerate the kind of shameless violence we saw throughout the course of the night,” St. Paul Mayor Chris Coleman told reporters Sunday after about 300 protesters had blocked Interstate 94 in St. Paul on Saturday night. “This doesn’t honor anyone’s memory.”

Minnesota Gov. Mark Dayton, a Democrat, called the shutdown of the interstate “unlawful and extremely dangerous.” He praised police and protest leaders “who were doing their utmost to stop this very dangerous situation,” news agencies reported.

The people arrested during the melee could be charged with rioting, the St. Paul police spokesman told The Post. The others, detained at a different location, were cited for public nuisance and unlawful assembly and released, he said.

Shortly before 10 p.m. local time Saturday, someone had shot at the San Antonio Police Department headquarters. No one was injured, but police leaders were anxious, given the slayings of five officers in Dallas on Thursday by a gunman who, police say, was enraged by the deaths of black people at the hands of police officers.

The unrest did not rise to the level of the widespread violence that erupted after the death of Freddie Gray in police custody in Baltimore last year of the shooting of Michael Brown in Ferguson, Missouri, in 2014. But demonstrations over the latest police-involved shootings were punctuated with tense confrontations.

In an interview Sunday morning on NBC’s “Meet the Press,” Ramsey said: “You can call it a powder keg. You can say that we’re handling nitroglycerin. But obviously when you just look at what’s going on, we’re at a very critical point in the history of this country.” The former police chief, who was chosen by Obama in 2014 to head the Task Force on 21st Century Policing, also expressed concern about incidents during the upcoming Republican and Democratic conventions because “the climate is simply too volatile.”

On Saturday, members of the New Black Panther Party came face to face with Louisiana state troopers outside a police headquarters. At night, police took dozens of activists in Baton Rouge into custody, including Mckesson.

Mckesson, who lives in Baltimore, documented protests in Baton Rouge on Saturday, his 31st birthday. He narrated the events surrounding him on Periscope, criticizing Baton Rouge police for what he saw as a heavy-handed response to peaceful protests. And did the same in frequent postings on Twitter and Vine.

As he and other activists marched through one of Baton Rouge’s busiest highways, which passes the police headquarters, Mckesson assailed police for provoking people after an officer threatened to arrest anyone walking on the road, according to video obtained by The Post. Another high-profile activist fired back that they were on the shoulder because there was no sidewalk.

Later, officers approached Mckesson. The smartphone he was using to broadcast the march and his ongoing commentary fell from his hands as he was arrested.

According to other activists, two police officers slammed Mckesson to the ground and took him into custody along with 33 other activists.

In a text message to The Post from within police custody, Mckesson said he and the nearly three dozen others were in custody together, wrists tied, and being taken to a police precinct. A police spokesman confirmed his arrest to the Advocate newspaper but did not elaborate on potential charges and did not respond to a request for comment from The Post.

Mckesson called a close friend in Baltimore around 5:30 a.m. and told her he was in OK physical condition but did not know when he would be released, the friend told The Post.

News of Mckesson’s arrest quickly spread on Twitter, fueling outrage over the possibility that he may have been deliberately targeted. The hashtag #FreeDeray began to trend almost immediately on Twitter after Mckesson’s arrest and was trending with more than 100,000 tweets hours later as of 5 a.m., with many tweets urging people to call Baton Rouge police and demand his release. Mckesson was arrested nearly a year ago in August during a sit-in outside a federal courthouse in St. Louis to commemorate the first anniversary of Brown’s death.

Mckesson rose to national prominence when he left Minneapolis after the death of Brown in Ferguson to become an activist and to document the growing movement seeking reforms in how law enforcement across the country treats communities of color. He has amassed roughly 450,000 Twitter followers and has been a forceful advocate for the Black Lives Matter movement on cable and in late-night television appearances.

Earlier this year, Mckesson sought to transform from activist to politician by running for Baltimore mayor. But the national spotlight wasn’t enough to endear him to voters there, and he finished far behind well-established political figures in the Democratic primary.

In the quiet period after the plethora of tense standoffs and arrests in Baton Rouge, three young protesters put on thin blue gloves and grabbed large trash bags.

“Last night, when people was running, they were tripping over the water bottles,” said Allyson Leach, 24. “This way if something happens, people will be safe.”

Across from them, police officers formed a human barricade outside police headquarters.

Still, protesters kept coming. They gathered on a swale outside a Shell gas station. They shouted, “No justice, no peace!”and wondered whether the police would charge at them again. They carried signs – “I Can’t Keep Calm I Have a Black Son,” one read – and raised their voices to sing “We Shall Overcome” and dropped used water bottles on to the ground.

Leach and her friends were there to pick them up.

“I am working full time, I’m a student and a mother,” added Shelby McKnight, 25. “But I am out here anyway because we need to stand up.”

Earlier Saturday evening in St. Paul, Castile’s friends, family and relatives from as far as St. Louis assembled in a parking lot at dusk, waiting for the light to fade.

Steps away, along Larpenteur Avenue in Falcon Heights, Castile was pulled over and fatally shot during a traffic stop on Wednesday, and his girlfriend streamed his dying moments and the officer’s reactions in a widely viewed Facebook Live video. In the days since, a makeshift memorial formed beside the pavement, where the hot sun wilted flowers and melted candle wax into puddles on the concrete sidewalk.

When the sunlight faded, family members lighted the remaining candles anew and tied up shiny balloons to memorialize Castile at the place where he died. His sister, Allysza Castile, thanked the group of about 50 who came to pay their respects. They held a moment of silence and prayed to “find justice in his death.”