Four Dallas police officers have been shot dead by snipers during protests against the shooting of black men by police, the authorities say.
An “intensive” search for suspects was under way, Dallas Police Chief David Brown said.
Gunfire broke out as demonstrators marched through the city, sending them fleeing for cover.
The protests were sparked by the deaths of Philando Castile in Minnesota and Alton Sterling in Louisiana.
Several more police officers were injured in the shooting, Chief Brown said, some of them critically.
The situation is still unfolding: there are reports one of the suspects has been cornered by police. They have issued a photo of one of those they are hunting.
he Dallas protests were among several held across the US over the police use of lethal force against African Americans.
Philando Castile was shot dead at a traffic stop in St Paul on Wednesday, while Alton Sterling killed by police a day earlier in Baton Rouge.
Both incidents were captured on video, reigniting what has become a national debate.
US President Barack Obama said “all fair-minded people should be concerned” about the frequent police killings of black Americans.
Pointing to statistics showing African-American citizens are far more likely to be shot by police by whites, Mr Obama called on law enforcement to root out internal bias.
“When incidents like this occur, there’s a big chunk of our fellow citizenry that feels as if it’s because of the colour of their skin, they are not being treated the same,” he said. “And that hurts