Police had taken three people into custody after the shootings on Thursday night and a standoff with another in a downtown garage, where gunfire had been exchanged, extended into Friday morning, officials said.
No motive has been given for the ambush at a downtown protest, one of many held in major cities across the US 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, according to Reuters.
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.
Police initially said four officers had been killed but the main union for Dallas police later reported that one of seven wounded officers had later died, taking the death toll to five.
The shooting happened as otherwise largely peaceful protests unfolded around the US after the shooting of Philando Castile, 32, by police near St. Paul, Minnesota, late on Wednesday.
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.