Five police officers have been killed in Dallas, USA and seven others wounded by gunmen during protests against the shooting of black men by police. Two civilians were also wounded in the attack.

Three people, including one woman, have so far been arrested in connection to the attacks, and one man who was in a stand-off with police was later killed.

Gunfire broke out at around 8:45 pm local time on Thursday as demonstrators marched through the city, protesting the killing of two Black-Americans by policemen earlier in the week.

The Dallas attack marks the deadliest day for US law enforcement officers since the September 11 attacks in 2001.

President Barack Obama, who is on a visit to Poland, said it was a “vicious, calculated and despicable attack on law enforcement”, adding that “anyone involved in these senseless murders will be held fully accountable”.

Dallas Police Chief, David Brown said the suspects were all believed to have been working together, including two snipers who fired from “elevated positions”, shooting some officers in the back.

“We believe that these suspects were positioning themselves in a way to triangulate on these officers from two different perches… and planned to injure and kill as many law enforcement officers as they could,” Chief Brown said.

Officers later surrounded a car park near El Centro College, as an armed man fired off rounds with a rifle.

Chief Brown said the suspect had told negotiators that “the end is coming” and that he was going to attack more officers and had “bombs all over the place”.

US media reported that the suspect later killed himself but the police chief clarified that he was later killed by a bomb robot which was detonated by the police.

Police are continuing to sweep the downtown area and no explosives have been found so far.

One of the officers killed was Brent Thompson, a 43-year old transport police officer and the first Dallas officer to be killed in the line of duty.

Statistics has it that 53 US officers have died in the line of duty in 2016, 21 of them as a result of gunfire, excluding those killed in Dallas.

Police had earlier issued a photo of one man at the rally with a rifle slung over his shoulder, saying he was a suspect.

The man, named as Mark Hughes, turned himself in to police and was later released.