DALLAS, July 8 - Dallas police were in a standoff with a suspect on Friday after snipers killed five officers and wounded six, one of the worst mass police shootings in U.S. history, during protests against the killing of two black men by police this week.

Police had taken three people into custody after the ambush shooting on Thursday night that police described as carefully planned and executed. Police were in a standoff that has extended into Friday morning with another suspect in a downtown garage, where gunfire had been exchanged, officials said.

White House officials have spoken with Dallas Mayor Mike Rawlings about the shooting that turned the downtown of one of the largest U.S. cities into a sprawling crime scene and unfolded along streets that house major corporations, restaurants and courthouses.

President Barack Obama, visiting Poland, has been briefed, a spokesman said.

No specific motive has been given for the sniper attacks at the downtown protest, one of many held in major cities across the United States on Thursday. New York police made more than a dozen arrests on Thursday night, while protesters briefly shut down one of Chicago's main arteries.

Dallas Police Chief David Brown said the shooters, some in elevated positions, used sniper rifles to fire at the officers in what appeared to be a coordinated attack.

"(They were) working together with rifles, triangulating at elevated positions in different points in the downtown area where the march ended up going," Brown told a news conference, adding a civilian was also wounded.

"It has been a devastating night. We are sad to report a fifth officer has died," Dallas police said on Twitter.

"Worst nightmare"

Mayor Rawlings advised people to stay away on Friday morning as police sort through the area where large areas have been cordoned off and transport halted. Federal authorities also halted commercial air traffic for the area as police helicopters hovered over the scene.

"Our worst nightmare has happened," the mayor said. "It is a heartbreaking moment for the city of Dallas."

The Dallas-Fort Worth metropolitan area is one of the nation's most populous and is home to more than 7 million people.

The use of force by police against African-Americans in cities from Ferguson, Missouri, to Baltimore and New York has sparked periodic and sometimes violent protests in the past two years and has spawned the Black Lives Matter movement.

Anger has intensified when the officers involved in such incidents have been acquitted in trials or not charged at all.

The shooting happened as otherwise largely peaceful protests unfolded around the United States after the shooting of Philando Castile, 32, by police near St. Paul, Minnesota, late on Wednesday. His girlfriend posted live video on the internet of the bloody scene minutes afterward, which was widely viewed.

The suspect in the Texas standoff told police "the end is coming" and that more police were going to be hurt and killed. Brown said the suspect also told police "there are bombs all over the place in this garage and downtown".

Police said they were questioning two occupants of a Mercedes they had pulled over after the vehicle sped off on a downtown street with a man who threw a camouflaged bag inside the back of the car. A woman was also taken into custody near the garage where the standoff was taking place.

"We are being very careful in our tactics so that we do not injure any of our officers or put them in harm's way. We still don't have a complete comfort level that we have all the suspects," Brown said.

"We are leaving every motive on the table on why this happened and how this happened," Brown said.

Rawlings later visited the wounded at Parkland hospital, the same hospital where President John F. Kennedy was taken after he was shot in Dallas in November 1963.

Television footage showed a heavy police presence, with officers taking cover behind vehicles on the street.

One of the officers killed has been identified. He is Brent Thompson, 43, and an officer with Dallas Area Rapid Transit.