Tensions remain sky high among police across the US after an ex army veteran used his military training to carry out a coordinated ambush which left five police officers dead.

Former corporal Micah Xavier Johnson used an AR-15 style rifle to target white officers at a Black Lives Matter protest in Dallas, Texas on Thursday night.

Horrifying footage emerged showing a police officer being executed at point blank range and another from a man who live streamed the shocking scenes.

25-year-old Johnson's massacre ended after an intense firefight with police when he was blown up by a robot sent in carrying a bomb.

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Three other people, two men and a woman, were arrested but officials have said Johnson was a lone gunman.

The victims have been named as Senior Cpl. Lorne Ahrens, 48, Officer Michael Krol, 40, Sgt. Michael J. Smith, 55, Officer Patrick Zamarripa and Dallas Area Rapid Transit (DART) police officer Brent Thompson, 43.

Racial tensions have been rising across the country after the shooting dead of several unarmed black men - the full background on that here.

But what is Black Lives Matter? And how has the movement taken off in the UK?