Dallas - More than 200 people were arrested in chaotic scenes during a new night of protests over US police violence towards blacks as authorities revealed on Sunday that the Dallas shooter had apparently been plotting a major bomb attack.Anger around America over the deaths of two black men at the hands of police last week - the state 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.Most of the protests on Saturday night into Sunday were peaceful. People inspired by the Black Lives Matter movement - which arose in recent years in response to repeated cases of 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 officer was hurt when a rioter dropped an 11kg chunk of concrete on his head from a bridge or overpass, police spokesperson Steve Linders said.It was in a Saint Paul suburb that one of last week's deaths occurred. Both killings were caught on horrific video that has since gone viral.In Baton Rouge, where the other black detainee died, more than 100 protesters were also arrested, local media reported citing police, among them the activist leader DeRay McKesson who live-streamed the incident.'Scrawled in his own blood' Chilling new details released about Dallas shooter Micah Johnson on Sunday fleshed out a still sketchy portrait of the 25-year-old US Army reservist and Afghanistan war veteran who apparently supported black militant organisations, some classified as hate groups.Police said the Dallas ambush shooter had taunted police negotiators and scrawled on a wall in his own blood before he was ultimately killed in the stand-off.He had opened fire on 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 Johnson's 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 the two black deaths last week were a trigger that prompted him to act, Dallas police chief David Brown told CNN on Sunday.Other plansInvestigators believe "based on evidence of bomb-making materials and a journal that the suspect had been practising 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 stand-off 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.Brown 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.Back to the 1960s? At one point earlier, Johnson, apparently wounded in an 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.Johnson also 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 calibre rifle, Brown said.The past week's violence - the black deaths, and then Dallas - 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.President Barack Obama has sought over the weekend to reassure Americans by insisting the United States can overcome its racial divisions, and rejecting comparisons with the civil unrest of the 1960s.Obama, scheduled to visit Dallas this week, described the gunman as a "demented individual" who in no way represented the African-American community.