Thousands of protesters took to the streets in US cities on Friday after a black extremist shot dead five cops in Texas during an otherwise peaceful march against police brutality. Five officers were shot dead in the late Thursday shooting, with seven other officers and two civilians wounded.
The shooting revives an emotional debate over lethal use of force by police, and problems of alleged police bias towards racial minorities, especially African-Americans.
Leaders of the Black Lives Matter protest movement condemned the Dallas violence, but vowed to uphold planned weekend marches. Vast crowds marched Friday in US cities including Houston, New Orleans, Detroit, Baltimore, and San Francisco. One of the largest was in Atlanta, where protesters blocked a major road.
In Phoenix, police in riot gear used pepper spray to disperse a large crowd blocking streets after rocks were hurled at them. At least one person was arrested. Addressing a prayer service honoring the fallen officers, Dallas mayor Mike Rawlings urged Americans to "step up" to heal the country's racial wounds.
"We will not shy away from the very real fact that we as a city, as a state, as a nation are struggling with racial issues," he told the crowd.
Rawlings echoed Obama's message that black lives matter -- and so do "blue" lives, those of police officers. "We must step up our game and approach complicated issues in a different way," Rawlings said. "And race is complicated."
'This must stop'
Obama, who ordered flags on government buildings lowered to half-mast for five days, said that there was "no possible justification" for violence against police.
The Dallas shootings sparked chaos as people ran for their lives during a march by several hundred demonstrators in the city of 1.2 million, near the site where president John F Kennedy was assassinated in 1963.
The peaceful protest was one of several nationwide over the deaths of Alton Sterling in Louisiana and Philando Castile in Minnesota that prompted Obama to make an emotional appeal for urgent police reform.
The ambush marks the single biggest loss of life for law enforcement in the United States since the September 11, 2001 terror attacks. "This must stop -- this divisiveness between our police and our citizens," Dallas police chief David Brown said.
'Every rabbit trail'
Johnson was killed in a tense showdown with police in a parking garage, by a bomb robot sent in by officers after hours of negotiations and an exchange of fire.
"This was a well-planned, well-thought out, evil strategy," said Brown of the gunman. "He said he was upset about the recent police shootings," Brown said. "The suspect said he was upset at white people. The suspect stated he wanted to kill white people, especially white officers."
Earlier, officials said three other people had been detained, but it was unclear if any remained in custody. US homeland security secretary Jeh Johnson said the gunman appeared to have acted alone.
Texas governor Greg Abbott said police would "continue down every rabbit trail... ensuring that we eliminate any other possible suspects or co-conspirators who may have aided this gunman in any way."