Snipers shot dead five police officers and wounded nine people, including two civilians, during a protest against police brutality in Dallas, in what US President Barack Obama on Friday called a "vicious, despicable and calculated" attack.

One suspect was killed in a showdown with police, Dallas mayor Mike Rawlings confirmed in interview with US media, while the other suspects in custody, including one black woman, were "not being real cooperative."

The shootings, which police described as a "terrorist incident," sparked chaotic scenes of people running for their lives during a march by several hundred demonstrators to protest the fatal shootings this week of two black men by police elsewhere in the country.

It 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.

One of the two civilians wounded was identified by local media 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.

(AFP)