'DESPICABLE ATTACK'

President Barack Obama, who was traveling in Poland, expressed his "deepest condolences" to Rawlings on behalf of the American people.

"I believe I speak for every single American when I say that we are horrified over these events and we are united with the people and police department in Dallas," he said.

Obama said the FBI was in contact with Dallas police and that the federal government would provide assistance.

"We still don't know all of the facts. What we do know is that there has been a vicious, calculated and despicable attack on law enforcement," he said.

The shooting, which erupted shortly before 9 p.m. CDT (0100 GMT), occurred near a busy area of downtown Dallas filled with restaurants, hotels and government buildings.

Mayor Rawlings advised people to stay away on Friday morning as police combed the area. Transportation was halted and federal authorities stopped commercial air traffic over the area as police helicopters hovered.

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

The Dallas shooting happened as otherwise largely peaceful protests unfolded around the United States after the police shooting of Philando Castile, a 32-year-old black man, on Wednesday during a traffic stop near St. Paul, Minnesota.

The day earlier, police in Baton Rouge shot dead another black man, Alton Sterling, 37, while responding to a call alleging he had threatened someone with a gun.

Over the last two years, there have been periodic and sometimes violent protests over the use of police force against African-Americans in cities from Ferguson, Missouri, to Baltimore and New York. Anger has intensified when the officers were acquitted in trials or not charged at all.

'THE END IS COMING'

The suspect in the Dallas standoff had told police "the end is coming" and that more police were going to be hurt and killed. Police chief 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 leaving every motive on the table on why this happened and how this happened," Brown said.

Mayor Rawlings 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.

Outside the hospital, officers stood in formation and saluted as bodies of the officers were about to be transported.