WASHINGTON/DALLAS (AFP) - The Dallas gunman is thought to have been plotting a major bomb attack, authorities said Sunday after 200 were arrested in a new night of protests over police violence against blacks.

Anger around America over the deaths of two black men at the hands of police last week - the stated reason for the black Dallas gunman's deadly rampage targeting white officers - showed no signs of abating with a prominent Black Lives Matter activist among those arrested.

US President Barack Obama cautioned protesting Americans against casting all police as racially biased, as the White House announced he would travel to Dallas Tuesday to address an inter-faith memorial service.

"If we paint police in broad brush without recognizing that the vast majority of police officers are doing a really good job... then we're going to lose allies in the reform cause," he said.

The past week's violence - the black deaths, and then Dallas - have shocked a country seemingly inured to its epidemic of gun violence and injected new urgency into the national debate on race relations and how largely white police forces deal with black suspects.

Most of the protests Saturday night into Sunday were peaceful. People inspired by the Black Lives Matter movement - which arose in response to police using lethal force against unarmed blacks - took to the streets in New York, Los Angeles and San Francisco.

But authorities said a full-scale riot broke out in Saint Paul, Minnesota and resulted in 102 arrests. Protesters blocked a freeway and attacked police with rocks, bottles, fireworks, Molotov cocktails and metal bars, police said.

Twenty-one officers were injured in the hours-long melee, one of them when a rioter dropped a 25-pound chunk of concrete on his head from a bridge or overpass, police spokesman Steve Linders said.

It was in a Saint Paul suburb that one of last week's deaths occurred.

In Baton Rouge, where the other death took place, more than 100 protesters were also arrested, local media reported citing police, among them the activist leader DeRay McKesson who livestreamed the incident.

Both killings were caught on horrific video that has since gone viral.

In another similar case gaining attention on social media, a black man was shot dead by police Saturday in Houston, the largest city in Texas. City police quoted by local media said that Alva Braziel was carrying a gun, and was believed to have pointed it at officers.

More than 500 people have died from US police bullets so far in 2016, according to data compiled by the Washington Post.

Chilling new details about Dallas shooter Micah Johnson Sunday fleshed out a still sketchy portrait of the 25-year-old US Army reservist and Afghanistan war veteran who apparently supported black militant organizations, some classified as hate groups, and died in the standoff with police.

Johnson opened fire Thursday evening with a powerful rifle during a peaceful protest against the shooting deaths of the two men in Louisiana and Minnesota, triggering hours of chaos in the downtown section of the big Texas city.

A search of his Dallas-area home turned up bomb-making materials and a manual in which he wrote about military tactics.

Police now believe he had been planning something big long beforehand, and that last week's deaths were a trigger, Dallas police chief David Brown told CNN on Sunday.

Investigators believe "based on evidence of bomb-making materials and a journal that the suspect had been practicing explosive detonations and that the materials were such that it was large enough to have devastating effects throughout our city and our north Texas area," Brown said.

"We're convinced that this suspect had other plans," he added.

The deaths in Minnesota and Louisiana "just sparked his delusion to fast-track his plans and (he) saw the protest in Dallas as an opportunity to begin wreaking havoc on our officers," Brown said.

Johnson toyed with police as he negotiated with them during a standoff after he first started shooting, Brown said.

"We had negotiated with him for about two hours. And he just basically lied to us, playing games, laughing at us, singing, asking how many did he get and that he wanted to kill some more," Brown said.

Johnson insisted on speaking only to a black police officer when he began negotiating, Brown said.

He knew the route of the Dallas march, and his military training apparently benefited him during the shooting, as he effectively triangulated police and started taking them out with his high caliber rifle, Brown said.

At one point earlier, Johnson, apparently wounded in the exchange of gunfire with police, wrote the letters "rb" in his blood on a wall at the community college where he had holed up.

Brown said it was not immediately clear what those letters meant.

The police chief said he told officers to improvise a plan because sharpshooters could not get a view of Johnson as he hid behind a brick wall. Brown accepted the idea of taking him out with a bomb carried by a police robot to within a few feet of Johnson.

"And I'll do it again if presented with the same circumstances," he said.