A suspect in the shootings in Dallas which left four police officers dead is exchanging gunfire with officers, the city’s police chief said.
David Brown said three people were now in custody, after a t least two snipers opened fire on police officers during protests in Dallas on Thursday night.
Seven other police officers were injured, while one member of the public was wounded in the gunfire.
Mr Brown told a news conference that the authorities are negotiating with a suspect in a central Dallas parking garage who has been exchanging gunfire with officials.
The chief said the suspect is not co-operating and has told negotiators he intends to hurt more law enforcement officials.
Mr Brown had earlier said that the snipers fired “ambush style” upon the officers.
Police said one suspect in the killings who was in a shoot-out with Dallas SWAT officers was in custody, while a second “person of interest” had surrendered. They said a suspicious package was being secured by a bomb squad.
Dallas police said they were questioning two occupants of a vehicle after an officer saw a person throw a bag into the back of the vehicle and speed off.
He spotted someone carrying a camouflage bag and quickly walking down the street. The person then threw the bag into the back of a black Mercedes and sped off.
Police said officers followed the vehicle southbound on Interstate 35 to a point south of Dallas where they performed a traffic stop. They then began questioning both occupants of the vehicle.
Television footage showed many police cars surrounding the vehicle.
Dallas police in uniform and in plainclothes were standing behind a police line at the entrance to the emergency room at Baylor Medical Centre in Dallas. It was unclear how many injured officers were taken there.
The gunfire broke out around 8.45pm local time on Thursday while hundreds of people were gathered to protest against fatal police shootings this week in Baton Rouge, Louisiana, and suburban St Paul, Minnesota.
The protesters had gathered after a Minnesota officer on Wednesday fatally shot Philando Castile while he was in a car with a woman and a child in St Paul.
The aftermath of the shooting was purportedly livestreamed in a widely shared Facebook video.
A day earlier, Alton Sterling was shot in Louisiana after being pinned to the pavement by two white officers. That, too, was captured on a mobile phone video.
Video footage from the scene showed that protesters were marching along a street about half a mile from City Hall when the shots erupted and the crowd scattered, seeking cover.
Texas Governor Greg Abbott said he has directed the Texas Department of Public Safety director to offer “whatever assistance the City of Dallas needs at this time”.
Mr Abbott said: “In times like this we must remember – and emphasise – the importance of uniting as Americans.”
The search for the gunman had stretched throughout central Dallas, 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.
“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 in the area, told the newspaper that the shooters “were strategic. It was tap tap pause. Tap tap pause.”
The gunshots in Dallas came amid other protests nationwide over the recent police shootings, which were peaceful.
In Manhattan, New York City, protesters first gathered in Union Square Park where they chanted “The people united, never be divided!” and “What do we want? Justice. When do we want it? Now!”
A group of protesters then left the park and began marching up Fifth Avenue blocking traffic during the height of rush hour as police scrambled to keep up.
Another group headed through Herald Square and Times Square where several arrests were reported.