Nationwide protests against police shootings turned violent Thursday night as two snipers, with the aid of multiple accomplices, executed a planned ambush during a Black Lives Matter protest in Dallas, killing five police officers and wounding seven others.

“It appears as if two snipers shot 10 police officers from elevated positions during the protest,” Dallas Police Department Chief David Brown said in a statement that he later amended to say that two other officers plus a civilian were also hurt.

Around 3 a.m. it was announced that the fifth officer had died. In addition to the 12 targeted officers, authorities said two civilians were also wounded.

In an earlier Friday news conference, Chief Brown said the snipers were part of at least a four-person team.

Three suspects were in custody and the fourth was pinned down in the El Centro parking garage in downtown Dallas before he was fatally shot by police.

The fourth suspect “has told our negotiators that the end is coming,” has threatened to kill more police and said there are “bombs all over the place in this garage and in downtown,” Chief Brown said before the suspect was killed.

The two snipers used rifles and were heavily armed, clad in masks and camouflage, and wore body armor — all sure signs of a planned ambush and not a confrontation that escalated. More than 50 shots were fired over several minutes, witnesses told Dallas news outlets.

President Obama, who is attending a summit in Warsaw, Poland, has been updated on the shooting of the police officers in Dallas.

“He asked his team to keep him updated on the situation as they get additional information,” said White House press secretary Josh Earnest.

“It’s a heartbreaking moment for the city of Dallas,” Dallas Mayor Mike Rawlings said at an earlier news conference.

Former New York City Police Commissioner Ray Kelly told ABC News that “we’ve never seen anything this scope and size since 9/11.”

At his early-morning news conference, Chief Brown said the three other suspects in custody includes a female who had been in the garage with the pinned-down suspect.

In addition, the Dallas PD said it had executed a traffic stop south of the area, in the Oak Cliff neighborhood, that is connected to the woman and to the ambush.

“A DPD officer observed an individual carrying a camouflaged bag, walking quickly down Lamar St. The individual threw the bag into the back of a black Mercedes” that sped off “at a high rate of speed,” the Dallas PD tweeted.

“Officers followed the vehicle southbound on I-35E and performed a traffic stop … police are questioning both occupants of the vehicle,” Dallas PD tweeted.

Police had released a photo of a camouflage-clad black man whom Chief Brown called a person of interest. But the man later turned himself in, and his brother told Dallas TV stations that police were mistaken.

The Dallas Area Rapid Transit Authority confirmed via Twitter that one of its officers had been killed and three others wounded, though they are expected to survive.

“The brave men & women of the Dallas Police and Sheriff’s Departments are in our thoughts and prayers. The Texas Attorney General’s Office will provide every measure of support in this difficult and developing situation,” Texas Attorney General Ken Paxton said on Twitter.

Mr. Rawlings, the mayor, said large parts of Dallas’s downtown area is an active crime zone with several blocks blocked off and that people should be wary about coming into work in the area. He advised residents to check DallasCityNews.net to check whether they will be able to get into their buildings.

The Dallas demonstrations, and several others around the country, were a response to two fatal office-involved shootings this week that were captured on videotape. The victims, both black men, were Alton Sterling in Baton Rouge, Louisiana, and Philando Castile in Minnesota.

In Washington D.C., hundreds of people spontaneously gathered at the White House and, without a permit but with the passive acquiescence of the police, took over Pennsylvania Avenue to head down to the U.S. Capitol.

One line began singing the civil-rights-era anthem “We Shall Overcome” in front of the U.S. Capitol around a police car as darkness descended on the nation’s capital.

But some demonstrators later shouted down Rep. John Lewis, Georgia Democrat when the civil-rights icon who was repeatedly beaten by Southern white cops in the 1960s appealed for peaceful demonstrations only.

Thousands of Black Lives Matter demonstrators marched through the rain in lower Manhattan on Thursday from Union Square and paralyzed one of the nation’s most-iconic neighborhoods in the largest of a series of marches over two police shootings of black men this week in Louisiana and Minnesota.

According to videos posted by the group Millions March NYC, chunks of Fifth and Sixth avenues had been shut down by 5,000 protesters “demanding an end to racist police murder” and Times Square had been “overrun and shut down.”

In St. Paul, Minnesota, more than 1,000 people gathered outside the governor’s official residence, prompting police to block vehicles from nearby streets.

Gov. Mark Dayton waded through the protesters as they chanted “What do we want? Justice. When do we want it? Now.” Mr. Dayton did not address the protesters, but earlier in the day he had opined that Mr. Castile wouldn’t have been shot had he been white.

In Chicago, according to cable-news footage, dozens of demonstrators locked arms and blocked an intersection of the city’s South Side near Interstate 94.

Thousands marched through the middle of Atlanta and there was a car-honking vigil outside the Baton Rouge store where Mr. Sterling was killed.

“Hey hey / Ho ho / These racist cops have got to go,” was one chant ringing out from the New York demonstrators.

Millions March New York also claimed a score of other issues against the police, accusing law enforcement of also hating women and gays, and also referring to “the colonizers’ Columbus Square.”

“34th and 8th is under the control of the people of NYC, not the racist, sexist, anti-queer NYPD who oppress us,” the group claimed.

At least 10 people were arrested when they sat down to block Times Square, prompting activists shutting down the area to begin cursing at the cops, according to a CNN reporter at the scene.

The Millions March New York Twitter feed accused the police of “making mult violent arrests, abt 35 so far-ppl literally ran over by police scooters #FTP.”

• Dave Boyer contributed to this article.

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