Organizers and supporters of the Black Lives Matter movement have condemned the slaying of police officers in Dallas during a protest against the fatal police shootings of black men in Louisiana and Minnesota.

In a statement posted online, the group - which has decried police violence against African Americans - deplored the videotaped deaths of Alton Sterling and Philando Castile this past week, and called for a transformation of policing in America.

In Dallas, the group noted, many had gathered seeking justice, when police were ambushed by snipers. A dozen officers were shot, five fatally, along with two civilians. One suspect was killed.

Dallas Police officials said it is unclear exactly how many snipers were involved. Three have been arrested in connected with the shooting spree. Mayor Mike Rawlings said the dead suspect, identified as Micah Xavier Johnson, 25, was slain by police using explosives in a parking garage where the man had exchanged gunfire with officers.

Police Chief David Brown said that, before dying, the suspect declared to officers that he was upset about recent shootings and wanted to kill whites, "especially white officers."

"The suspect said he was upset about Black Lives Matter," Brown told reporters. "He said he was upset about the recent police shootings. The suspect said he was upset at white people. The suspect said he wanted to kill white people."

The suspect had also said he was not affiliated with any groups and that he acted alone, Brown added.

"There are some who would use these events to stifle a movement for change and quicken the demise of a vibrant discourse on the human rights of Black Americans," the Black Lives Matter group wrote. "We should reject all of this.

"Black activists have raised the call for an end to violence, not an escalation of it. Yesterday's attack was the result of the actions of a lone gunman. To assign the actions of one person to an entire movement is dangerous and irresponsible. We continue our efforts to bring about a better world for all of us."

Supporters of #BLM have also condemned the violent attacks on police.

New York Daily News columnist Shaun King said on Twitter that he hates police brutality but doesn't hate police. He called yesterday's violence "wrong on every level."

King also says anyone blaming Black Lives Matter for the police officers' murders "is sick":

Malkia Cyril, director of the Center for Media Justice, says her "heart hurts for the dead."

Cyril also defended the Black Lives Matter movement, writing that it "advocates dignity, justice and freedom, not the murder of cops."

Former NBA player Jay Williams tweeted:

Police brutality & hate retaliation need to stop but whatever we do, we must do it in an orderly, peaceful, non violent fashion. #Dallas

-- Jay Williams (@RealJayWilliams) July 8, 2016

However, a law enforcement source told CBS News that monitoring of social media overnight indicated an increase of threats to police online, with one post saying "kill cops," CBS News senior investigative producer Pat Milton reports.

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