President Obama issued a call for unity on Saturday, insisting that race relations in the US were not returning to the dark days of the Sixties.

Mr Obama delivered his message as nationwide protests against police brutality towards black Americans have continued to spill over into violence in the aftermath of the massacre in Dallas.

"There is sorrow, there is anger, there is confusion about next step, but there's unity in recognising that this is not how we want our communities to operate,” he said. "When we start suggesting somehow that there's this enormous polarisation, and we're back to how we were in the Sixties, that's just not true."

Mr Obama, who has announced he is abandoning a European visit and will arrive in Dallas in the coming days in a bid to calm race relations, called gunman Micah Johnson “demented” and insisted he in no way represents America’s black community.

All clear after part of city locked down following security threat

Police have lifted a lockdown imposed on an area around their headquarters following a security threat.

Around a square mile was blocked off with people inside the security cordon forbidden from leaving after an alert, while police searched a car park.

The scale of the response was a reflection of the edginess in the city following the murder of five or its officers.

Details of the threat were not disclosed by the police who deployed SWAT teams and closed down the public transit station close to their HQ.

At one point police used a rifle to force their way into part of the car park. However an exhaustive search using officers and dogs failed to find a suspect and the all clear was given.