"If you listen real close, you can hear the people in power shaking," David Hogg, a survivor who has emerged as one of the student leaders of the movement, told the roaring crowd of demonstrators at the March for Our Lives rally in Washington.
He warned: "We will get rid of these public servants who only care about the gun lobby."
March for our Lives Rally in NYC:
Chanting "Vote them out!" and bearing signs reading "We Are the Change," ''No More Silence" and "Keep NRA Money Out of Politics," the protesters packed Pennsylvania Avenue between the Capitol and the White House.
Large rallies with crowds estimated in the tens of thousands in some cases also unfolded in such cities as Boston; New York; Los Angeles; Chicago; Houston; Fort Worth, Texas; Minneapolis; and Parkland, Florida, the site of the Feb. 14 attack at Marjory Stoneman Douglas High School that left 17 people dead.
Protesters denounced the National Rifle Association and its allies and complained that they are scared of getting shot in school and tired of inaction by grown-ups after one mass shooting after another.
PHOTOS: NYC March For Our Lives 2018
They called for such measures as a ban on high-capacity magazines and assault-type rifles like the one used by the Florida killer, tighter background checks and school security, and a raising of the age to buy guns.
"I'm really tired of being afraid at school," said Maya McEntyre, a 15-year-old high school freshman from Northville, Michigan, who joined a march by thousands in Detroit. "When I come to school, I don't want to have to look for the nearest exit."
She added: "I want to get to the problem before it gets to me."
In Atlanta, Ben Stewart, a 17-year-old senior at Shiloh Hills Christian School in Kennesaw, Georgia, took part in a march in Atlanta to press for what he called "common-sense gun laws."
"People have been dying since 1999 in Columbine and nothing has changed. People are still dying," Stewart said. "It could be prevented."
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President Donald Trump was in Florida for the weekend. A motorcade took him to his West Palm Beach golf club in the morning. As of early afternoon, he had yet to weigh in on Twitter about the protests.
The NRA went silent on Twitter in the morning, in contrast to its reaction to the nationwide school walkouts against gun violence March 14, when it tweeted a photo of an assault rifle and the message "I'll control my own guns, thank you."
About 30 gun-rights supporters staged a counter-demonstration in front of FBI headquarters in Washington, standing quietly with signs such as "Armed Victims Live Longer" and "Stop Violating Civil Rights."
Organizers of the gun-control rally in the nation's capital hoped their protest would match in numbers and spirit last year's women's march, which far exceeded predictions of 300,000 demonstrators.
"We will continue to fight for our dead friends," Delaney Tarr, another survivor of the Florida tragedy, declared from the stage. The crowd roared with approval as she laid down the students' central demand: a ban on "weapons of war" for all but warriors.
The Rev. Martin Luther King Jr.'s 9-year-old granddaughter Yolanda Renee King gave a rousing speech at the Washington rally, drawing from the civil rights leader's most famous words.
"I have a dream that enough is enough," she said. "That this should be a gun-free world. Period."
In Parkland, the police presence was heavy as more than 20,000 people filled a park near the school, chanting slogans such as "Enough is enough" and carrying signs that read "Why do your guns matter more than our lives?" and "Our ballots will stop bullets."
Gun violence was also fresh for some in the Washington crowd: Ayanne Johnson of Great Mills High in Maryland held a sign declaring, "I March for Jaelynn," honoring Jaelynn Willey, who died Thursday two days after being shot by a classmate at the school. The classmate also died.
Rallying outside the New Hampshire Statehouse in Concord, 17-year-old Leeza Richter said: "Our government will do more to stop us from walking out than it will to stop a gunman from walking in."
Since the bloodshed in Florida, students have tapped into a current of gun control sentiment that has been building for years - yet still faces a powerful foe in the NRA and its supporters.
Organizers hope the passions of the crowds and the under-18 roster of speakers will translate into a tipping point starting with the midterm congressional elections this fall. In addition to pushing for tighter gun laws, the students have been working to register young people to vote.
Polls indicate public opinion in the U.S. may be shifting on the issue.
A new poll conducted by The Associated Press-NORC Center for Public Affairs Research found that 69 percent of Americans think gun laws in the U.S. should be tightened. That is up from 61 percent in 2016 and 55 percent in 2013.
Overall, 90 percent of Democrats, 50 percent of Republicans and 54 percent of gun owners now favor stricter gun laws.
At the same time, the poll found that nearly half of Americans do not expect elected officials to take action.
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