​Washington: A three-day spasm of violence and protests in cities across the US climaxed Thursday night with the killing of five cops and the wounding of six more in Dallas, Texas by what police said were "two snipers [shooting] from elevated positions" during a protest over the latest white-cop/black- victim killings.

As chaos engulfed Dallas and one of the suspected gunmen was holed up in a garage – reportedly negotiating with officer. Another three suspects were in custody - a woman detained near the parking garage where the sniper is pinned down and two men in a Mercedes leaving the area.

Police Chief David Brown said that a civilian also had been wounded; with three officers in a critical condition and two others in surgery.

Claiming the snipers had also planted a bomb which police had yet to locate in the downtown area, he said: "[They] planned to injure and kill as many law enforcement officers as they could."

In a second media briefing, Chief Brown said the sniper had told negotiators "the end is coming" and "there were bombs all over the place".

Earlier, police had released a picture of a man they believe to be one of the gunmen. The police tweeted: "This is one of our suspects - please help us find him!" The man was later cleared of involvement.

Describing the protester-police interaction as peaceful, a witness Carlos Harris said the lethal shooting had been strategic – "it was tap, tap, pause. Tap, tap, pause."

According to The Dallas Morning News, the shooting started at about 9pm, as hundreds gathered near the intersection of the city's Market and Main streets, close to a Memorial Plaza that marks the assassination of President John F Kennedy in 1963.

Video on social media showed people fleeing from the protest group as it marched from the Belo Garden Park towards the Old Red Courthouse. Similarly it was a viral social media response to two killings by police gunmen that sparked the Dallas protest.

It was one of dozens across the country after the killing of Alton Sterling, 37, in the early hours of Tuesday as he was pinned on the ground outside a convenience store in Baton Rouge, Louisiana; and the killing of Philando Castile, 32, in St Paul, Minnesota, after he was pulled over for a faulty tail light while driving home on Wednesday evening.

Before the attack on the cops in Dallas, President Barack Obama urged Americans to understand the statistics of why people of colour believe they are not treated the same as whites and Minnesota state Governor Mark Dayton challenged the death of Castile, asking : "Would this have happened if the driver were white, if the passengers were white?"

Answering his own question, Governor Mark Dayton told a news conference: "I don't think it would have."

In Warsaw, Poland, Obama told reporters: "These are not isolated incidents. They're symptomatic of a broader set of racial disparities that exist in our criminal justice system and I just want to give people a few statistics to try to put in context why emotions are so raw around these issues."

Obama said in he was restricted in what he might say about the Baton Rouge death because the Justice Department's civil rights division has taken the lead on the investigation, but then he offered much of the same statistical data quoted by activists protesting police violence.

"African Americans are 30 per cent more likely than whites to be pulled over. After being pulled over, African Americans and Hispanics are three times more likely to be searched. Last year, African Americans were shot by police at more than twice the rate of whites.

"African Americans are arrested at twice the rate of whites; African Americans defendants are 75 per cent more likely to be charged with offenses carrying mandatory minimums. They receive sentences that are almost ten percent longer than comparable whites arrested for the same crime."

Obama said that if tallying figures explained why the African-American and Hispanic population, 30 per cent of the general population, "make up more than half of the incarcerated population."

And in the expectation of a backlash that now is guaranteed in the wake of the police killings in Dallas, Obama urged: "To be concerned about these issues is not political correctness. It's just being an American and to recognise the reality that we got some tough history and we haven't gotten through that history yet."

Speaking about the protests, Obama posed a troubling question: "I would just ask those who question the sincerity or the legitimacy of protests and vigils and expressions of outrage, who somehow label those expressions of outrage as 'political correctness,' I just ask folks to step back and think," he said. "What if this happened to somebody in your family? How would you feel?"

Follow Fairfax World on Facebook

Follow Fairfax World on Twitter