Dallas was gripped by a new security scare Saturday triggered by an anonymous threat in the Texas city, on edge days after a gunman fatally ambushed five police officers during a peaceful protest.
SWAT teams deployed around the Dallas Police Department headquarters while officers investigated reports of a suspicious person in a parking garage -- finally giving the all-clear around two hours later.
Police took "precautionary" security measures across the city after receiving "an anonymous threat against law enforcement," the Dallas police said in a statement.
The scare came as another night of marches against police brutality was underway in several US cities, a groundswell of protest that shows little sign of abating.
Protesters led by the Black Lives Matter movement are demanding justice for two African-Americans shot dead by police this week -- their dying moments captured in viral video footage that stunned the nation.
At the Dallas protest late Thursday, a 25-year-old black army veteran named Micah Johnson used a rifle to shoot dead five police officers in a sniper attack. Seven other cops were wounded, as well as two civilians.
Johnson told negotiators before police killed him that he wanted to murder white cops in revenge for the black deaths.
Dallas officials believe he was the lone shooter in the incident.
Police across the country were on edge as it emerged that officers had been targeted in at least two incidents -- in Tennessee and Wisconsin -- by individuals apparently angered at the recent fatal shootings of black men by police.
Hundreds of people marched peacefully Saturday in New York for a third consecutive night, holding up banners bearing the names of Alton Sterling and Philando Castile, the two men whose deaths, in Louisiana and Minnesota, triggered the latest protests.
In St. Paul, where Castile was killed, several hundred protesters blocked a highway intersection for about three hours and hurled rocks and bottles at police, who were equipped with helmets, clubs and gas masks.
The officers used smoke grenades, pepper spray and tear gas to break up the crowd, and around midnight arrested protesters who refused to move.
In San Francisco, a large force of police swooped in to prevent protesters, who marched for a second day, from blocking a major road intersection.
Hundreds also marched in Los Angeles, including in South Central, the epicenter of violent 1992 riots following the acquittal of white police officers in the videotaped beating of black motorist Rodney King.
There were nasty scenes late Friday in Phoenix, Arizona, where police used pepper spray to disperse stone-throwing protesters. And in Rochester, New York, 74 people were arrested over a sit-in protest.
But elsewhere -- from Atlanta to Houston, Chicago, New Orleans, Boston, Detroit and Baltimore -- weekend protests over the fatal shootings have passed off with little trouble.
President Barack Obama tried Saturday to reassure a shocked country, insisting that the United States can overcome its racial divisions, and rejecting comparisons with the civil unrest of the 1960s.
Obama, scheduled to visit Dallas next week, described the gunman as a "demented individual" who in no way represented the African-American community.
"I firmly believe that America is not as divided as some have suggested," he said during a NATO summit in Warsaw. "There is sorrow, there is anger, there is confusion... but there is unity."
US politicians have sought to appear as unifiers after the week of violence.
"White Americans need to do a better job of listening when African Americans talk about the barriers they face," Democratic White House hopeful Hillary Clinton tweeted.
That message was echoed by prominent members of the Republican Party, which has often jumped to defend law enforcement amid accusations of racial bias.
"It is more dangerous to be black in America," said Newt Gingrich, a Republican former House speaker tipped as a possible White House running mate for Donald Trump.
"Sometimes it's difficult for whites to appreciate how real that is. It's an everyday danger."
There has been a huge surge of sympathy for Dallas police after what marked the single biggest loss of life for US law enforcement since the September 11, 2001 terror attacks.
Although the White House has ruled out any link between Johnson and known "terrorist organizations," his Facebook page ties him to radical black movements listed as hate groups.
Police found bomb-making materials and a weapons cache at Johnson's home and were scouring his journal and social media posts to understand what drove him to mass murder.