Dallas - The Dallas mass shooting that has claimed the lives of five police officers, and at least one suspected sniper after a tense standoff, has been dubbed the deadliest day for law enforcement since September 11, 2011.Snipers shot dead five officers and wounded six others during a protest against police brutality in Dallas, in what US President Barack Obama on Friday called a "vicious, despicable and calculated" attack.A suspect was reportedly killed after a protracted standoff during which police traded fire with the suspect at a downtown garage. CNN cited a police source as saying one suspect was killed, although some local media outlets said he had shot himself.CNN also reported that a SWAT team of elite police marksmen and bomb-sniffing dogs had been deployed to the scene and stun grenades were used. Police have since described the shootings as a "terrorist incident".Biggest single loss of lifeIt was the biggest single loss of life for law enforcement in America since the September 11, 2001 attacks, and appeared likely to further strain already tense race relations in the country.Police Chief David Brown said earlier that the suspect had "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."Three other Dallas suspects were taken into custody – a woman and two men found with camouflage bags in a car, though Brown had warned there were likely more suspects.Brown said police believed at least two snipers had shot at police ambush-style from high points during an otherwise peaceful protest against police shootings of black men, after two African Americans – Alton Sterling in Louisiana and Philando Castile in Minnesota – were killed this week.Thousands marched in protest in Atlanta, Chicago, Los Angeles, Saint Paul, Washington and other cities late on Thursday, with more than 1,000 protesters gathering in New York's Time Square.Another man turned himself in to 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. His lawyers said he had already received thousands of death threats.Complete pandemoniumA civilian was also wounded. Local media identified her as Shetamia Taylor, 37, saying she was shot in the leg while attending the protest with her four sons.One witness at the rally spoke of "complete pandemonium," in an area close to the site where president John F Kennedy was killed in 1963."There was blacks, whites, Latinos, everybody. There was a mixed community here protesting. And this just came out of nowhere," Cory Hughes, a protest organiser, told CNN."I'm still kind of startled, shaken up. As you know being in the front, it's almost like the gunshots were coming at us. It was complete pandemonium... It's bananas."Dramatic video of the shooting emerged from witnesses, who posted the footage online. Bursts of gunfire and police sirens could be heard in the videos.Ismael DeJesus, who filmed the attack while hiding in the Crown Plaza Hotel, described how one of the gunmen shot an officer on the ground. "It looked like an execution, honestly. He stood over him after he was already down. Shot him maybe three or four more times in the back," he told CNN.Witness Ismael Dejesus joins @donlemon to share the video he captured of the #Dallas shooting https://t.co/oBmEIv09rk— CNN (@CNN) July 8, 2016In another video, posted by Twitter user @allisongriz, one witness can be heard saying: "Oh, my God. There are people laying on the ground. I hope they're just hiding."'Heartbreaking morning'Mayor Mike Rawlings spoke of a "heartbreaking morning" for Dallas. "We as a city, we as a country, must come together, lock arms and heal the wounds that we all feel from time to time. Words matter. Leadership matters at this time," he said.Brown initially said two of the injured officers were undergoing surgery and three were in critical condition. The condition of the other officers was unclear.Among the officers killed was Dallas Area Rapid Transit officer Brent Thompson, 43, the agency said, adding he was the first DART officer killed in the line of duty.Lynn Mays was at the scene when the shooting started. He said an officer saved his life. pic.twitter.com/Kc4aXlg9GT— Caleb Downs (@Calebjdowns) July 8, 2016Outside Parkland Hospital, police saluted their fellow officers who lost their lives or were wounded in the shooting. Other people later joined the officers for an impromptu vigil, their hands grasped behind their backs."Just because we say black lives matter doesn't mean blue lives don't matter," Obama had said overnight after arriving in Warsaw for a Nato summit.