At least five police officers have been shot dead in the United States at protests over recent police killings of black men.
The Dallas Police Department said on Twitter: "It has been a devastating night. We are sad to report a fifth officer has died."
At least three of those killed were shot by what appeared to be sniper fire, police said.
Addressing the media, Dallas police chief David O Brown said the force was in negotiation with one suspect, and that gunfire was being exchanged on the second storey of a central parking garage. That suspect, he added, was "not being very cooperative".
Three other people were in custody, he said, including a woman.
"The suspect we are negotiating with ... has told our negotiators that the end is coming and he's going to hurt and kill more of us, meaning law enforcement and that there are bombs all over the place in this garage and downtown, so we are being very careful with our tactics," Brown said.
"We still don't have a complete comfort level that we have all the suspects."
Al Jazeera's Heidi Zhou-Castro, reporting from Dallas, said: "This [the standoff] is right in the heart of downtown Dallas. The march happened at what's called the Belo Garden Park. It's a little green space surrounded by tall office buildings where the communities have rallies of this type. The parking garage overlooks that park."
Gunfire broke out late on Thursday during an otherwise peaceful protest over two recent police killings of black men in Minnesota and Louisiana. The sound of gunfire sent marchers running and police scrambling for cover.
Reports said hundreds attended the protests.
Brown said two gunmen shot at police officers from "elevated positions", hitting at least 11 of them. At least three of the wounded were in a critical condition, he said.
Firefighters and police were keeping people away as dozens of police cars with their lights flashing converged at the scene.
The Federal Aviation Administration issued a temporary flight restriction over the city.
US President Barack Obama was being briefed on the shooting of police officers, while on a visit to Poland for a NATO summit, the White House said.
'Everyone started running'
"Everyone just started running," Devante Odom, 21, told The Dallas Morning News. "We lost touch with two of our friends just trying to get out of there."
Carlos Harris, who lives downtown, told the newspaper that the gunmen "were strategic. It was tap pause".
Video footage from the scene showed protesters marching along a street in the city centre, about half a mile from City Hall when the shots erupted and the crowd scattered, seeking cover.
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 emphasise - the importance of uniting as Americans," he said, in a statement.
The search stretched throughout downtown, an area of hotels, restaurants, businesses and some residential apartments. The scene was chaotic, with helicopters hovering overhead and officers with automatic rifles on the street corners.
Speaking ahead of the violence, Robert James, an organiser of the protest, told Al Jazeera he had expected fewer people to attend.
"I thought this was going to be me and 300 friends... I, like everyone else, felt like I couldn't sit back," he said, referring to the two recent killings of black men by police. "It's the straw that broke the camel's back."
The Dallas Police Department said on Twitter: "It has been a devastating night. We are sad to report a fifth officer has died."
At least three of those killed were shot by what appeared to be sniper fire, police said.
Addressing the media, Dallas police chief David O Brown said the force was in negotiation with one suspect, and that gunfire was being exchanged on the second storey of a central parking garage. That suspect, he added, was "not being very cooperative".
Three other people were in custody, he said, including a woman.
"The suspect we are negotiating with ... has told our negotiators that the end is coming and he's going to hurt and kill more of us, meaning law enforcement and that there are bombs all over the place in this garage and downtown, so we are being very careful with our tactics," Brown said.
"We still don't have a complete comfort level that we have all the suspects."
Al Jazeera's Heidi Zhou-Castro, reporting from Dallas, said: "This [the standoff] is right in the heart of downtown Dallas. The march happened at what's called the Belo Garden Park. It's a little green space surrounded by tall office buildings where the communities have rallies of this type. The parking garage overlooks that park."
Gunfire broke out late on Thursday during an otherwise peaceful protest over two recent police killings of black men in Minnesota and Louisiana. The sound of gunfire sent marchers running and police scrambling for cover.
Reports said hundreds attended the protests.
Brown said two gunmen shot at police officers from "elevated positions", hitting at least 11 of them. At least three of the wounded were in a critical condition, he said.
Firefighters and police were keeping people away as dozens of police cars with their lights flashing converged at the scene.
The Federal Aviation Administration issued a temporary flight restriction over the city.
US President Barack Obama was being briefed on the shooting of police officers, while on a visit to Poland for a NATO summit, the White House said.
'Everyone started running'
"Everyone just started running," Devante Odom, 21, told The Dallas Morning News. "We lost touch with two of our friends just trying to get out of there."
Carlos Harris, who lives downtown, told the newspaper that the gunmen "were strategic. It was tap pause".
Video footage from the scene showed protesters marching along a street in the city centre, about half a mile from City Hall when the shots erupted and the crowd scattered, seeking cover.
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 emphasise - the importance of uniting as Americans," he said, in a statement.
The search stretched throughout downtown, an area of hotels, restaurants, businesses and some residential apartments. The scene was chaotic, with helicopters hovering overhead and officers with automatic rifles on the street corners.
Speaking ahead of the violence, Robert James, an organiser of the protest, told Al Jazeera he had expected fewer people to attend.
"I thought this was going to be me and 300 friends... I, like everyone else, felt like I couldn't sit back," he said, referring to the two recent killings of black men by police. "It's the straw that broke the camel's back."