Dallas Police Headquarters On Security Alert After Threat Against Officers

Dallas police headquarters temporarily heightened security on Saturday after receiving a threat against officers, two days after a gunman killed five officers in the Texas city.

"The Dallas Police Department received an anonymous threat against law enforcement across the city and has taken precautionary measures," Dallas police said in an emailed statement to media.

Though police officers told AFP the headquarters were on lockdown, the department tweeted that that was not the case.

SWAT officers were deployed at the main building, local media reported.

The sight of a masked man caused police to go on heightened alert, the Dallas Morning News reported.

"A man wearing a black mask was spotted in a parking garage behind the headquarters," the paper said.

"Officers have completed manual search of the garage. No suspect found," the police department tweeted.

"Great job by our officers!" police chief David Brown tweeted after an all-clear was issued soon after.

The alert came two days after a gunman killed five officers and wounded seven others in a sniper-style attack during a peaceful protest in Dallas against police brutality toward African Americans.

The demonstrations were part of nationwide protests over the fatal police shootings of two black men this week in Louisiana and Minnesota.

The Dallas ambush on Thursday marked the single biggest loss of life for law enforcement in the United States since the September 11, 2001 terror attacks.

Police were set further on edge on Friday as it emerged that several officers had been targeted across the country by individuals apparently angered at the recent fatal shootings.

The nightmare scenes in Texas left many fearing a new, dark chapter in America's troubled race relations.

Thousands marched in US cities overnight on Friday, and there were nasty scenes 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.

At Dallas police headquarters on Friday, people had flocked to leave flowers and messages of sympathy for the slain officers amid an outpouring of support that saw crowds in the city line up to hug police officers.

But elsewhere, fresh protests were planned against police brutality in at least half a dozen cities on Saturday.