Snipers kill five police at US protest over shootings

The gunfire broke out late on Thursday as hundreds of people were gathered to protest this week's fatal police shootings in Louisiana and Minnesota, causing scenes of chaos.

Three people are in custody and a fourth suspect was exchanging gunfire with police in a parking garage in downtown, Dallas, Police Chief David Brown said early on Friday morning.

"[The suspect] has told our negotiators that the end is coming, and he is going to hurt and kill more of us, meaning law enforcement. And that there are bombs all over the place in this garage and in downtown," Brown told reporters.

"The snipers fired ambush style upon the officers from elevated positions during the protest."

Brown added that it appeared the shooters "planned to injure and kill as many officers as they could".

	The shootings prompted marches across the US [Getty]

A woman who had been in the same part of the garage was also in custody, along with two suspects who had camouflage bags in a Mercedes and a person of interest who turned himself in after police tweeted a picture of him wearing camouflage and an assault rifle slung over his shoulder.

The search for the shooters stretched throughout downtown, an area of hotels, restaurants, businesses and some residential apartments. The scene was chaotic, with helicopters hovering overhead and officers with automatic rifles on the street corners.

"We still don't have a complete comfort level that we have all the suspects," Brown warned.

Dramatic video of the shooting emerged from witnesses, who posted the footage online. Bursts of gunfire and police sirens could be heard in the videos.

The deaths of Alton Sterling in Louisiana and Philando Castile in Minnesota are once again fuelling debate about police use of lethal force, especially against blacks.

Police in the spotlight

The deaths of Alton Sterling in Louisiana and Philando Castile in Minnesota are once again fuelling debate about police use of lethal force, especially against blacks.

Castile's girlfriend Diamond Reynolds live-streamed the aftermath of Wednesday night's shooting in a Saint Paul suburb with an officer pointing his gun at her through the window as her four-year-old daughter sat in the back of the car.

The 10-minute video - which shows Castile bleeding profusely - prompted widespread outrage and has been viewed millions of times after it was posted on Facebook.

It prompted thousands to march across Los Angeles, Chicago, Washington, Dallas and Atlanta on Thursday evening, with more than 1,000 protesters gathered in New York's Time Square.

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Agencies contributed to this report.