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A dramatic protest late Saturday and early Sunday on Interstate 94 in St. Paul turned violent, with 21 police officers injured and more than 100 people arrested. Under occasional clouds of colored smoke, fireworks, hurled rocks and tear gas, confrontations continued until 4:30 a.m. Sunday, as demonstrators responded to the police killing in Falcon Heights of Philando Castile.

Just a few hours later, St. Paul Mayor Chris Coleman and Police Chief Todd Axtell stood together and denounced as "a disgrace" the pelting of officers with rocks, bottles and other items.

At an 8 a.m. news conference, Axtell said 21 officers from all law enforcement agencies on the scene were injured in the mayhem. One of them suffered a broken vertebra after a concrete block was dropped on his head; he was still hospitalized as of Sunday afternoon, police said. The State Patrol said six of the 21 were troopers who were slightly hurt by thrown objects. Steve Linders, St. Paul police spokesman, said he doesn't know how many other officers are still in the hospital.

The all-night faceoff on a major freeway drew national attention. President Obama also weighed in Sunday, saying at a news conference during a one-day stop in Madrid that "any violence directed at police officers is a reprehensible crime."

Obama added, "Whenever those of us who are concerned about failures of the criminal justice system attack police, you are doing a disservice to the cause."

Gov. Mark Dayton, whose Summit Avenue residence has been the site of a protest encampment, said Sunday that "the occupation and shutting down of Interstate 94 last night were unlawful and extremely dangerous. … I urge all Minnesotans to remain calm and peaceful during this very difficult time."

Roughly 50 of the arrests came during the "freeway riot," which began shortly before nightfall Saturday and continued until police cleared the freeway after 1 a.m., allowing the interstate to reopen in both directions, Linders said.

State Patrol spokesman Sgt. Troy Christianson said those arrested were likely to be held in the Ramsey County jail until Monday.

"More than 20 verbal warnings" to disperse were issued before the apprehensions began, the State Patrol said in a statement, adding that 49 adults and one juvenile could face charges of third-degree riot, a gross misdemeanor.

Another 50 or so arrests were made about 4 a.m. Sunday near Grand Avenue and Dale Street. Those demonstrators were cited with misdemeanors at the scene and released, Linders said.

The officers were hurt from demonstrators "throwing rocks, bottles, fireworks and bricks," Linders said. Demonstrators were seen on a pedestrian overpass throwing objects including bricks and rebar at officers and dumping liquid on them.

At Sunday's news conference at police headquarters, the mayor called the violence "shameful" and said it "doesn't honor anyone's memory … including Philando Castile's."

Echoing that sentiment was Philando's mother, Valerie Castile. "When demonstrations become violent, it disrespects my son and his memory," she said in a statement Sunday. "Philando was a man of peace and dignity. Please, I ask you to at all times remain peaceful in your expressions of concern regarding his death at the hands of the police. I promise that we will not rest until justice prevails."

Axtell began his comments by holding up a chunk of concrete as an example of what was thrown at his officers.

"This is the first time in my 28 years we have observed this level of violence toward our public servants," Axtell said. "It's really a disgrace."

The chief said the protesters "turned into criminals. I am absolutely disgusted, [and] I am not going to tolerate it. This is just something that doesn't happen in St. Paul."

Activists converged Sunday afternoon for another march, this one starting at the St. Anthony Police Department. The officer who shot Castile is a member of that city's police force.

Unrest in the Twin Cities drew the head of the national NAACP to Minnesota. Cornell Brooks met with Dayton and other local leaders Sunday afternoon and also spoke at Progressive Baptist Church in St. Paul, calling for laws to counter racial profiling by police.

About 10 p.m. Saturday, officers had issued their 16th order to vacate the interstate, and protesters were not budging. At one point, two dozen officers in riot gear pressed on the crowd, a paddy wagon following behind.

As police advanced on I-94 near Lexington Parkway, some among the hundreds of protesters retreated up an adjacent hill and left the freeway. Some people began to march back to the tents-and-tarps encampment at the governor's residence, where demonstrators have kept a constant presence since shortly after Castile's death.

About 11:15 p.m., police began arresting people one by one, escorting them to a repurposed Metro Transit bus. Officers in riot gear used smoke bombs and pepper spray to disperse the crowd.

In the heat of the protest, as many as 300 people were spread across both the eastbound and westbound lanes, and about 200 police officers were present. State Patrol squad cars had the interstate blocked for 5 miles in both directions and entrance ramps sealed off from Hwy. 280 east to downtown St. Paul.

State Patrol Col. Matt Langer, joining the mayor and police chief at the Sunday morning news media briefing, said, "Protesters have ample safe areas to make their voices heard. The freeway is not one of these options."

About 9 p.m. on Twitter, Black Lives Matter Minneapolis posted: "We shut down 94 for Philando. We are gonna need bail money. Please make a gift now."

The protest quickly grew into the day's biggest and most disruptive demonstration in the Twin Cities, following a peaceful daylong gathering at the governor's residence and a separate rally at Loring Park that spilled into the streets of downtown Minneapolis, where protesters briefly stopped traffic at 9th Street and Hennepin Avenue, blocked an entrance at the Basilica Block Party and marched past Target Center.

Castile, a 32-year-old school cafeteria supervisor from St. Paul, was fatally shot by a police officer Wednesday night during a traffic stop in Falcon Heights. The aftermath, as he lay dying in the driver's seat, was livestreamed on Facebook by his girlfriend sitting alongside him while her 4-year-old daughter sat in the back seat.

The girlfriend, Diamond Reynolds, said Castile was shot for no reason. The officer, Jeronimo Yanez, said through his attorney that Castile was displaying a handgun and was warned to comply with the officer's orders.

Star Tribune staff writers Claude Peck, Randy Furst and Karen Zamora contributed to this report.

Paul Walsh • 612-673-4482