Snipers shot dead five police officers and injured six others in Dallas, unleashing chaos during a protest against police shootings of black men in an ongoing standoff Friday.

Snipers shot dead five police officers and injured six others in Dallas, unleashing chaos during a protest against police shootings of black men in an ongoing standoff Friday.

Police had the city on lockdown after a suspect warned that bombs had been planted throughout the city center.

The shootings, which the police called a "terrorist incident", took place as several hundred people marched through the Texan city to protest the fatal shootings this week of two black men by police in Louisiana and Minnesota.

As the rally was winding up, shots rang out around 9:00 pm (0200 GMT Friday), causing panic among the protesters, who scrambled to take cover.

City police chief David Brown said two snipers shot at the police "from elevated positions during the protest/rally". A civilian was also wounded.

Police continued to trade fire with a suspect into the early hours of Friday at a downtown garage.

The suspect "has told our negotiators that the end is coming, and he is going to hurt and kill more of us, meaning law enforcement. And that there are bombs all over the place in this garage and in downtown," Brown told reporters.

Three others suspects were taken into custody -- one woman and two men found with camouflage bags in a car.

"We still don't have a complete comfort level that we have all the suspects," Brown warned.

Another man turned himself into the police after the authorities tweeted a picture of him wearing camouflage and an assault rifle slung across his shoulder, with a call for information on his whereabouts.

It is legal for those with permits to openly carry weapons in the state of Texas. The man, named as Mark Hughes, was later released.

The protest was one of several nationwide over the fatal police shootings of black men in Minnesota and Louisiana this week that have prompted President Barack Obama to make an emotional appeal for urgent police reform.

The Federal Aviation Administration restricted use of the airspace over the city center until 1130 GMT, saying "only relief aircraft operations under direction of Dallas Police Department are authorized."

Rail and bus links were suspended in the downtown area of the usually bustling Texas city.