Carrying signs bearing the names of blacks who allegedly died at the hands of white policemen, protesters who marched from the U.S. Capitol to the Justice Department building stage a die-in on Dec. 8. (Kate Patterson/For The Washington Post)

Across the country, people outraged over the police killings of unarmed black men have flowed into the streets using many of the same tactics and chanting many of the same slogans, holding up their hands in a gesture of surrender and staging “die-ins.” From Oakland to New York, they have even donned the same T-shirts.

But for all the similarities, the burgeoning social movement that began four months ago in Ferguson, Mo., remains a fractured and disparate effort, with no large national organization coordinating or guiding its trajectory. And while experts warn that a diffuse movement may ultimately fail, many of the protesters say they like it this way.

And so it was a homegrown effort that led a clutch of protesters in the District to block major thoroughfares around the White House and Dupont Circle during rush hour Monday. It was local clergy members in Philadelphia who drew about 200 protesters to a “die-in” outside Lincoln Financial Field on Sunday, where they lay prone as people streamed out of the Eagles football game. And local activists in California led the marches that ended in violence in Oakland and Berkeley over the weekend.

“We very purposefully have been a leaderless movement. . . . We’ve been doing that intentionally because we know absolute power corrupts absolutely,” said Brittany Packnett, who was among a small group of Ferguson protesters who met with President Obama last week. “That ensures we remain about the heart of the movement rather than certain personalities.”

The next few months could test that approach as pressure grows for protesters to turn their expressions of anger and grief into concrete policies in the form of state laws and congressional action. Often, it is the tough going of policymaking that causes loose-knit movements to fall apart, said Martin Berger, a professor of art history at the University of California at Santa Cruz who has written two books on the civil rights movement.

Chanting "This is what democracy looks like" and "I can't breathe," protesters took over Forever 21 in Times Square and H&M in Manhattan on Sunday. (Sarah Parnass/The Washington Post)

“The lack of movement leaders is both a strength and a weakness,” Berger said. “It is a strength in that these are grass-roots movements of people coming together with a lot of different grievances. But that lack of leadership often leads to a fracturing down the road when there is disagreement among the participants and when it becomes difficult to move change.”

On Monday, Obama addressed the minority of cases where there has been violence. “As long as they’re peaceful, I think they’re necessary,” he said of the protests in an interview with Black Entertainment Television. “When they turn violent, they’re counterproductive.”

In Berkeley, a five-hour march on Saturday was broken up with tear gas after hundreds of protesters clashed with police. Authorities said protesters, many of them anarchists, vandalized police cars, and hurled pipes and bricks at officers. Also on Saturday, Seattle police arrested seven protesters who were throwing rocks, according to reports.

“A country’s conscience has to be sometimes triggered by inconvenience,” Obama said. He added that “the value of peaceful protests, activists, organizing is it reminds the society this is not yet done.”

A large protest may be on the horizon in the District. The “National March Against Police Violence” on Saturday at Freedom Plaza is being organized by the National Action Network, the group founded by the Rev. Al Sharpton.

Sharpton’s group, more than any other, has been at the forefront of this new movement against police killings. In an interview, he said the value of a well-connected national organization should not be discounted.

“The police have unions that provide them with legal support and coordinate media, whereas families don’t have that,” he said. “We’re like the union for families because they want an infrastructure to fight an infrastructure. You can’t sit up in your living room and fight an institution unless you have institutional support.”

As the movement turns its sights to achieving changes — for example, setting up a division within the Justice Department to specifically deal with police-brutality cases and broadening the use of body cameras by police — it helps to have an experienced, established organization leading the charge, some advocates say.

“We do not want this to be an episodic movement,” Sharpton said. “We want it to be real change.”

But some among the new, younger crop of activists have rejected this idea — and have rejected Sharpton.

“The only time you see him is on the TV,” said Pierre Lawson, 26, a nonprofit worker from the District who organized some of the road blockages in the city. He accused Sharpton of self-promoting and fanning the flames of racial strife. Still, he acknowledged, “it would be nice to have some sort of national group.”

Others have served to rally the troops nationally, including the parents of the men killed and Benjamin Crump, a Florida lawyer representing several of the families. Crump has been compared to Johnnie L. Cochran Jr., the lawyer who represented O.J. Simpson who was also known for speaking out against police brutality.

Many of the protesters say that while there may not be a national group, there has certainly been national coordination, helped along by social media. Several of the protest leaders from Ferguson flew to New York last week after a grand jury there decided not to file charges against a police officer in the death of an unarmed African American man who was selling cigarettes. Eric Garner died after the officer used a chokehold in an effort to arrest him.

Protesters have been copying one another’s tactics. For example, the “die-in” — when people lie down en masse — has become a common method of anti-police-brutality demonstration across the country. Protesters have worn shirts reading “I can’t breathe,” Garner’s final words. Monday night, Cleveland Cavaliers star LeBron James wore such a T-shirt while warming up before a basketball game against the Nets in Brooklyn.

In Cleveland, protesters have been demonstrating against the Nov. 22 shooting of Tamir Rice, a 12-year-old killed by an officer responding to a 911 call about someone with a gun at a park. Rice had been playing with a BB gun. Samaria Rice, the boy’s mother, said Monday that she wanted the officer who killed her son convicted.

Social media fills some of the gaps left by a traditional leader or organization, so it makes sense that some young people would reject the idea of a person or an organization as a standard-bearer, said Matthew C. Whitaker, director of the Arizona State University Center for the Study of Race and Democracy.

They prefer “leadership that’s shared and that doesn’t rely so much on one major leader, because history has shown we kill those leaders,” he said. “We marginalize them. We undermine their reputation. We do something to undermine their ability to execute and lead. And, as a result, vacuums are left and the movement crumbles.”