Five police officers were injured.
On the Republican side, presumptive GOP presidential nominee Donald Trump edged away from his earlier calls for unity, blasting Obama and presumptive Democratic nominee Hillary Clinton and tweeting that America is "a divided nation".
In Minnesota, 150-200 protesters shut down a section of Interstate 94, St. Paul spokesman Steve Linders said.
They followed similar protests across the United States after police shot dead two black men - Alton Sterling on Tuesday in Louisiana and Philando Castile on Wednesday in Minnesota. A retired New York City police detective wept as he spoke to CNN's Chris Cuomo about the bravery of the Dallas officers who, carrying only pistols, were wearing protective vests they knew couldn't deflect the bullets of the shooter's weapon. In open-carry states like Louisiana, some protesters marched with guns displayed.
In Atlanta, a group of protesters confronted police on the 17th Street bridge, over Interstates 75 and 85, then moved down Peachtree Street, the city's main thoroughfare, into the Midtown business district.
Hundred protesters in St Paul blocked a highway intersection for several hours late Saturday and hurled firecrackers, rocks and bottles at police.
Police said one suspect they had engaged in a shootout had been arrested, and a bomb squad unit was investigating a suspicious package found near the suspect's location.
The rally led into a panel discussion on justice, policing, inequality and other social issues.
She urged supporters to check Oklahoma's Black Lives Matter Facebook Event Page frequently over the weekend.
"No police officer should act brutally", he continued. "You're doing a disservice to the cause" with heated rhetoric, Obama said.
"I want to say to say to everyone concerned about... racial bias in the criminal justice system that maintaining a truthful, serious and respectful tone is going to help mobilize American society to bring about real change", Obama said.
"This week people felt hurt and angry".
"The "Black Lives" Matter" demonstration saw protestors march to a police station and block buses and traffic, an AFP photographer at the scene said. What is happening to this country? And I think there's a target on police officer's backs because of groups like Black Lives Matter that make it seem like all police are against blacks. Johnson said before the police killed him that he wanted to murder white cops in revenge for the black deaths. Chamberlain, who was Britain's prime minister from 1937 to 1940, advocated a policy of appeasement in response to Nazi Germany's aggression before World War II.
Almost a day later in Lancaster, a "Standing with Black Lives Vigil" originally scheduled to mourn both Sterling and Castile went ahead as scheduled with a minimal police presence, it appeared.
A second "person of interest" had turned himself in, they said, although there was no word on the arrest of the second sniper.
Sky's Tom Parmenter said: "One of the striking things is how connected this all is, people on their smartphones sharing photos and videos of this protest around the world". Johnson cited federal grants and training that "support the efforts of the local police in terms of surveillance, communications, first-responder equipment, equipment to protect their own safety, their own lives".