Five police officers have died and six others were injured by snipers on Thursday during protests in Dallas that called for justice after the police shootings in Louisiana and Minnesota this week.

One suspect exchanged gunfire with police, and was arrested early on Friday morning, police announced on Twitter

Police Chief David Brown said perhaps up to six snipers ambushed law enforcement, striking 10 officers and a civilian.

“Working together with rifles, (they) triangulated at elevated positions in different points in the downtown area where the march ended up going,” Brown said.

Police said the shootings occurred during a protest rally, which drew a crowd of 800, over the recent police shootings of two black men, Alton Sterling and Philandro Castile.

Dallas Mayor Mark Rawlings said downtown Dallas remains an active crime scene. Rawlings called the shooting Dallas’ “worst nightmare,” Thursday night, and asked that all residents “come together and support our police officers.”

Some witnesses said the shooter or shooters ambushed officers from a multi-story parking garage.

“The cops had no idea who was shooting at them,” Jamal Johnson told KTVT-TV in Dallas. “Everyone knew it wasn’t a firework — it was an actual shot.”

Brown, the police chief, said that it appeared the attackers “planned to injure and kill as many officers as they could”.

Texas Governor Greg Abbott called for unity. “In times like this we must remember – and emphasize – the importance of uniting as Americans,” he said, in a statement.

The shooting broke out just hours after President Obama reacted to the shootings deaths of black men in Minnesota and Louisiana and called upon all Americans to confront persistent racial disparities between law enforcement and the communities they serve.

“This is not just a black issue. This is not just a Hispanic issue. This is an American issue,” Obama said. The fatal police shootings are “symptomatic of a broader set of racial disparities that exist in our criminal justice system.”

The shootings of two black men, Alton Sterling in Baton Rouge, Louisiana, and Philando Castile in Falcon Heights, Minnesota. The shootings reignited outrage over the police treatment of black Americans; both instances were illustrated by graphic videos.