DEVELOPING: Dallas Police confirmed shortly before midnight local time that both suspects in the murder of four police officers were in custody, with one of the suspects captured after a shootout with SWAT members. A suspicious package discovered near that suspect's location was being secured by bomb squad members.
A man who police identified as a "person of interest" had turned himself in, but it was not yet known what his relationship was to the shooting.
Dallas Police Chief David Brown confirmed the four fatalities — three Dallas officers and one transit officer — and said seven other officers and one civilian were wounded in the shooting. Three of the injured officers reportedly were in critical condition and two others were in surgery.
Dallas' public transit agency, DART, confirmed on Twitter that one of its officers was shot and killed, while three of its officers suffered non-life-threatening injuries.
Brown said the shooters positioned themselves in two parking garages in downtown Dallas and "planned to injure and kill as many law enforcement officers as they could."
"It's a heartbreaking moment for the city of Dallas," Mayor Mike Rawlings said. "I ask everybody focus on one thing right now, and that is Dallas police officers, their families, those that are deceased [and] those that are in the hospital fighting for their lives."
Fox4 cameras captured protesters running away from the scene of the shooting shortly before 9 p.m. local time. A cameraman approached the scene and captured officers apparently lying on the ground.
#BREAKING: Our cameras captured several shots ring out during a protest in Downtown Dallas pic.twitter.com/OWOBOOI8Jg
— FOX 4 NEWS (@FOX4) July 8, 2016
A Dallas police source estimated to Fox News that at least 60 rounds were fired over a "large kill zone." The source added that the shooting would have required considerable planning.
Aerial images showed officers appearing to focus their search on a nearby parking garage, which may have served as the snipers’ perch.
Witness Carlos Harris told the Dallas Morning News the gunfire was "strategic. It was tap-tap-pause. Tap-tap-pause."
Texas Gov. Greg Abbott released a statement saying he has directed the Texas Department of Public Safety director to offer "whatever assistance the City of Dallas needs at this time."
"In times like this we must remember -- and emphasize -- the importance of uniting as Americans," Abbott said.
The protesters had gathered after a Minnesota officer on Wednesday fatally shot Philando Castile while he was in a car with a woman and a child in a St. Paul suburb. The aftermath of the shooting was purportedly livestreamed in a widely shared Facebook video.
A day earlier, Alton Sterling was shot in Louisiana after being pinned to the pavement by two white officers. That, too, was captured on a cellphone video.
Michael Bautista told KDFW the protest march had been "very peaceful" before the shooting started.
Brittany Peete, a demonstrator, told the Associated Press she didn't hear the gunshots, but she "saw people rushing back toward me saying there was an active shooter."
Peete said she saw a woman trip and nearly get trampled as people ran to get to safety.
"Everyone just started running," Devante Odom, 21, told The Dallas Morning News. "We lost touch with two of our friends just trying to get out of there."
Fox News' Bret Baier and the Associated Press contributed to this report.