Protesters took to the streets in cities across the United States late Friday to demand justice for the African American victims of police violence after a similar peaceful rally in Dallas a night earlier had turned violent.

A sniper attacked police who were guarding a Black Lives Matter march in the Texas city late Thursday, killing five officers, wounding seven officers and two civilian bystanders.

Police identified the lone gunman as Micah Xavier Johnson, 25, a black military veteran who told officers he was angry about the killing of black men by police officers and wanted to kill police.

A search of Johnson's home turned up bomb making materials, ballistic vests, rifles, ammunition, and a personal journal of combat tactics.

The incident had prompted a wave of national grief and along with the police killing of two black men earlier in the week renewed a discussion about police brutality and racial violence in the United States.

Vice President Joe Biden said the killings had "touched the soul of the nation" as he called for unity amid heightened racial tensions.

"As Americans, we are wounded by all of these deaths," Biden said in the White House's weekly radio address, noting that the police had been shot while protecting a peaceful protest against police killings of black men.

Meanwhile thousands of Black Lives Matter marchers disrupted traffic in Atlanta and demonstrators also descended on New York's Times Square and the Justice Department in Washington, television images showed. In New Orleans, protesters lay down in a "die in" in front of police headquarters, the Times-Picayune newspaper reported.

Police had increased security in cities across the country amid fears of copy-cat attacks.

In Georgia, police were investigating an incident where a police officer was apparently ambushed after responding to calls about a car break-in. The police officer then shot his assailant. There was no apparent connection to the Dallas shooting, police said. Both were expected to survive.

US President Barack Obama was cutting short a trip to Europe to return home on Sunday to deal with the attack and travel to Dallas next week.

The attack just before 9 pm Thursday (0200 GMT Friday) began as officers were mowed down by sniper fire as the peaceful demonstration ended. A street-level shootout ensued, and police exchanged gunfire for hours with Johnson.

Police said they killed him with a bomb carried by a robot into the parking garage where he took cover.

The slain police officers were guarding a demonstration by activist group Black Lives Matter protesting the killing of two black men by police elsewhere in the country earlier in the week.

Alton Sterling, 37, was shot and killed in Baton Rouge Tuesday and Philando Castile, 32, in suburban Minneapolis Wednesday.

The attacks also renewed calls by Obama and Democrats for tougher gun laws, which the Republican congressional majority have refused to discuss.

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