US President Barack Obama is to travel to Dallas, Texas, on Tuesday to speak at a memorial service for five police officers killed Thursday by a sniper at a protest against police shootings of black men.

Obama will travel to Dallas at the invitation of Mayor Mike Rawlings, White House spokesman Josh Earnest said Sunday in a statement.

Dallas Police Chief David Brown said earlier in an interview with CNN that the shooter had planned an even bigger attack.

Investigators found explosives in Micah Johnson's flat, as well as diary entries hinting at the intended scale of the violence and his tactical movements, Brown told the broadcaster.

Police are convinced Johnson had other plans and that he believed what he was doing was justified. Brown said he was determined to "make us pay for what he [saw] as law enforcement's efforts to punish people of colour."

The 25-year-old, who investigators believe acted alone, was killed when police sent a robot to detonate a bomb in the car park where he was holed up. He had previously told officers that he wanted to kill "white people, especially white police officers."

Brown defended the police' decision to detonate the bomb and kill Johnson.

"Without our actions, he would have hurt more officers," Brown said.

The police chief said the suspect had shot at police, while also laughing, singing and threatening further violence.

He said he appreciated critics, "but they're not on the ground, their lives are not being put at risk by debating what tactics to take."

Activists continued to protest the recent race-related killings overnight Saturday, and many of the rallies descended into scuffles with police officers over the weekend.

In the city of Saint Paul, Minnesota, police and marchers from the Black Lives Matter movement clashed during a protest over the shooting to death of Philando Castile by police in the area.

Saint Paul Police Department said on Twitter that 21 officers were injured by protesters throwing missiles during a stand-off on an interstate highway.

Over 100 people were arrested on charges ranging from public nuisance and unlawful assembly to third degree rioting.

Brown said his message to demonstrators across the country who are protesting police shootings of black men is: "We are sworn to protect you and your right to protest, and we will give our lives for it."

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