MINNEAPOLIS - Tens of thousands of protesters took to the streets across America again Sunday, with peaceful demonstrations against police killings of black people overshadowed by unrest that quickly ravaged parts of cities from Pennsylvania to California.

City and state officials had deployed thousands of National Guard soldiers, enacted strict curfews and shut down mass transit systems, but that did little to stop many cities from again erupting into unrest.

Protesters in Philadelphia hurled rocks and Molotov cocktails at police, officials said, while masked crowds broke into upscale stores in a San Francisco suburb, fleeing with bags of merchandise. In Minneapolis, a truck driver drove into a massive crowd of demonstrators nearly a week after George Floyd died after pleading for air as an officer pressed a knee into his neck.

At least 4,100 people have been arrested over days of protests since Floyd's death Monday, according to a tally compiled by The Associated Press. Arrests ranged from looting and blocking highways to breaking curfew.

In Salt Lake City, a leading anti-police brutality activist condemned the destruction of property but said broken buildings shouldn't be mourned on the same level as black men like Floyd.

“Maybe this country will get the memo that we are sick of police murdering unarmed black men,” said Lex Scott, founder of Black Lives Matter Utah. “Maybe the next time a white police officer decides to pull the trigger, he will picture cities burning.”

For a second day, the protests reached to the White House, where chants could be heard from around 1,000 demonstrators just across the street in Lafayette Park as they faced police in riot gear behind barricades. The scene was defiant but peaceful, though police used flash bangs to stop another group from reaching the park.

As the protests grew, President Donald Trump retweeted conservative commentator Buck Sexton who called for “overwhelming force” against violent demonstrators.

Yet thousands still marched peacefully, with some also calling for an end to the fires, vandalism and theft, saying it weakened calls for justice and reform.

“They keep killing our people,” said Mahira Louis, 15, who marched with her mother and several hundred others through downtown Boston. “I'm so sick and tired of it.”

The officer who pressed his knee onto Floyd's neck has been charged with murder, but protesters are demanding the other three officers at the scene be prosecuted. All four were fired.

“We're not done,” said Darnella Wade, organizer for Black Lives Matter in neighbouring St. Paul, where thousands gathered peacefully in front of the state capitol. “They sent us the military, and we only asked them for arrests.”

Minnesota Gov. Tim Walz said Sunday that Attorney General Keith Ellison will take the lead in prosecutions in Floyd's death.

Hennepin County Attorney Mike Freeman said earlier that he had asked Ellison, who is black, to help. Freeman, who is white, has been criticized by civil rights activists and some city officials, who say there is a history of mistrust between his office and members of the community.

Walz told reporters that Ellison “needs to lead this case.” He said he made the decision after speaking with Floyd's family who “wanted to believe that there was a trust, and they wanted to believe that the facts would be heard.”

Walz brought in thousands of National Guard soldiers to help quell violence that had damaged or destroyed hundreds of buildings in Minneapolis over days of protests. The immense deployment there appeared to have worked Saturday night, when there was comparatively little destruction.

On Sunday, in a display of force, long lines of state patrolmen and National Guard soldiers were lined up in front of the Capitol, facing the demonstrators, with perhaps a dozen military-style armoured vehicles behind them.

Disgust over generations of racism in a country founded by slaveholders combined with a string of recent racially charged killings to stoke the anger. Adding to that was angst from months of lockdowns brought on by the coronavirus pandemic, which has disproportionately hurt communities of colour, not only in terms of infections but in job losses and economic stress.

The droves of people congregating for demonstrations threatened to trigger new outbreaks, a fact overshadowed by the boiling tensions.

The scale of the protests, sweeping from coast to coast and unfolding on a single night, rivaled the historic demonstrations of the civil rights and Vietnam War eras.

Curfews were imposed in major cities around the U.S., including Atlanta, Chicago, Denver, Los Angeles, San Francisco and Seattle. About 5,000 National Guard soldiers and airmen were activated in 15 states and Washington, D.C.

In tweets Sunday, Trump blamed anarchists and the media for fueling the violence. Attorney General William Barr pointed a finger at “far left extremist” groups. Police chiefs and politicians accused outsiders of coming in and causing the problems.

At the Minneapolis intersection where Floyd was killed, people gathered with brooms and flowers, saying it was important to protect what they called a “sacred space.” The intersection was blocked with the traffic cones while a ring of flowers was laid out.

Among those descending on Minneapolis was Michael Brown Sr., the father of Michael Brown, whose killing by a police officer in Ferguson, Missouri, set off unrest in 2014.

“I understand what this family is feeling. I understand what this community is feeling,” he said.

In Indianapolis, two people were reported dead in bursts of downtown violence, adding to deaths reported in Detroit and Minneapolis in recent days.

Buildings around the U.S. were defaced with spray-painted messages, from the facade of St. Patrick's Cathedral in New York to the historic Hay-Adams hotel near the White House. Some of Floyd's gasped last words - “I can't breathe” - were repeated, alongside anti-police messages.

Sedensky reported from Philadelphia. Associated Press journalists across the U.S. contributed to this report.