DALLAS (AFP) – Snipers shot dead five police officers and wounded seven others in Dallas during a protest against police brutality in the wake of two fatal shootings of black men in other US cities, the city's mayor said Friday.

Two civilians were also shot and wounded in the chaotic fray late Thursday in the sprawling Texas city, Mike Rawlings said in interview with US media.

President Barack Obama called it a "vicious, calculated and despicable attack on law enforcement" and pledged that those responsible would be held accountable.

One suspect was killed in a tense showdown with police in a parking garage, he confirmed, while the other three suspects in custody, including one black woman, were "not being real cooperative."

It was the single biggest loss of life for law enforcement in America since the September 11, 2001 attacks, and appeared likely to further strain already tense race relations in the country.

"They put their lives on the line, lost their lives for the safety of citizens, exercising their right to speech. And it is a terrible tragedy," Rawlings told CNN.

"I don't know why we have to constantly be angry at each other. (...) We've got to make sure that we love each other this way, and not argue with each other."

Rawlings explained that police used an explosive device sent in by robot during the standoff with the suspect, who he only described as "male."

"He was saying he was going to take everybody out. He threatened other bombs and we felt that was the safest way to get in and it was," he said.

Sweeps have been completed and no explosives were found, Dallas police major Max Geron said on Twitter.

Pandemonium

One witness at the rally spoke of "complete pandemonium," in an area close to the site where president John F Kennedy was killed in 1963.

"There was blacks, whites, Latinos, everybody. There was a mixed community here protesting. And this just came out of nowhere," Cory Hughes, a protest organizer, told CNN. "I'm still kind of startled, shaken up. As you know being in the front, it's almost like the gunshots were coming at us. It was complete pandemonium... It's bananas."

Earlier, Police Chief David Brown said three suspects had been taken into custody – a woman and two men found with camouflage bags in a car, though Brown had warned there were likely more suspects.

He said police believed at least two snipers had shot at police ambush-style from high vantage points.

"Just because we say black lives matter doesn't mean blue lives don't matter," Obama had said overnight in support of police after arriving in Warsaw for a NATO summit.

Downtown Dallas was on lockdown, with no bus or rail service and flight restrictions.

Outside Parkland Hospital, police saluted their fellow officers who lost their lives or were wounded in the shooting.

Other people later joined the officers for an impromptu vigil.

The Dallas protest was one of several nationwide over the deaths of Alton Sterling in Louisiana and Philando Castile in Minnesota that have prompted Obama to make an emotional appeal for urgent police reform.

Thousands marched in Atlanta, Chicago, Los Angeles, Saint Paul, Washington and other cities late Thursday, with more than 1,000 protesters gathering in New York's Times Square.