(CNN) Survivors of the deadly shooting rampage at a Parkland, Florida, high school led hundreds of thousands Saturday in March for Our Lives events across the country, delivering a resounding message that Washington's inaction on the scourge of gun violence is no longer acceptable.

Building on the momentum of last week's National School Walkout , these members of a generation raised with gun violence have mobilized Americans with impassioned pleas for stricter gun control laws while honoring the 17 students and faculty members killed February 14 at Marjory Stoneman Douglas High School.

"To the leaders, skeptics and cynics who told us to sit down, stay silent and wait your turn, welcome to the revolution," Marjory Stoneman Douglas student Cameron Kasky told the throngs in Washington, where the march turned into a thunderous, standing-room-only rally.

"Either represent the people or get out. Stand for us or beware."

View of the Washington March for Our Lives from the roof of the Newseum.

Though Washington hosted the main event, more than 800 sister marches were held across the country, from Boston to Los Angeles, and around the world. Students, teachers, parents, survivors of school shootings and celebrities took their defiant message against gun violence and the gun lobby to the seats of power.

'Hear the people in power shaking'

Kasky, who was among the Washington speakers, read the names of classmates and teachers who died in Parkland. The list ended with Nicholas Dworet, who would have turned 18 on Saturday.

"Nicholas, we are all here for you," he said. "Happy birthday."

Parkland student David Hogg told the massive crowd near the Capitol that "you can hear the people in power shaking" with the approach of midterm elections.

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"They've gotten used to being protective of their position, the safety of inaction," he said, before issuing a warning to candidates supported by the National Rifle Association.

"To those politicians supported by the NRA that allow the continued slaughter of our children and our future, I say get your resumes ready."

With tears streaming, Parkland survivor Emma Gonzalez told the Washington crowd that the massacre lasted about six minutes, 20 seconds. She remembered her fallen classmates. She enumerated some ordinary things they will never do again.

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"Since the time I came out here, it has been 6 minutes 20 seconds, and the shooter has ceased shooting and will soon abandon his rifle, blend in with the students as they escape and walk free an hour before arrest," she said.

"Fight for your lives before it is someone else's job."

The NRA posted a message Saturday on Facebook, saying the March for Our Lives was orchestrated by "gun-hating billionaires and Hollywood elites ... manipulating and exploiting children as part of their plan to DESTROY the Second Amendment and strip us of our right to defend ourselves and our loved ones."

From Washington to Utah, smaller groups of counterprotesters gathered in support of the Second Amendment.

'Life isn't equal for everyone'

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Naomi Wadler, 11, a student from Virginia, said she spoke for all African-American girls lost to gun violence whose stories were ignored by the media. And she warned against dismissing her message because of her age.

"We might still be in elementary school, but we know," she said. "We know life isn't equal for everyone. And we know what's right and wrong. We also know that we stand in the shadow of the Capitol. And we know that we have seven short years until we, too, have the right to vote."

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Yolanda Renee King , the 9-year-old granddaughter of the Rev. Martin Luther King Jr., told the crowd that she, too, has a dream: "Enough is enough."

The youth-led movement, started in the aftermath of the massacre with media appearances by survivors such as Kasky and Gonzalez, has drawn celebrity support.

Singer Andra Day joined Baltimore's Cardinal Shehan School Choir on stage to sing "Rise up" for the Washington crowd.

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Ariana Grande, Miley Cyrus, Demi Lovato, Common and Lin-Manuel Miranda also participated.

Chants of "We want change!" rose over the capital after a closing performance by singer Jennifer Hudson, whose mother, brother and nephew were shot and killed in 2008 by her former brother-in-law.

The Parkland survivors and others students thanked the crowd at the end.

"We are united," a young woman said. "Congress, politicians -- you are parents. Hear your children cry."

Said another, "We are magic. We are power."

Parallels to civil rights marches

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In New York, former Beatle Paul McCartney told CNN he marched because his friend and bandmate John Lennon was lost to gun violence in 1980 not far from where the crowd had assembled.

"This is what we can do," he said, "so I'm here to do it."

Earlier, a normally bustling swath of Manhattan went quiet during a moment of silence as the names of the Parkland victims were read.

When it over, Mayor Bill de Blasio tweeted, "Amazing! Final NYC tally: 175,000 joined #MarchForOurLives. And this is a movement that has JUST BEGUN. These students WILL change America!"

Amazing! Final NYC tally: 175,000 joined #MarchForOurLives. And this is a movement that has JUST BEGUN. These students WILL change America! — Bill de Blasio (@NYCMayor) March 24, 2018

In Washington, a teenager drew parallels with the civil rights marches of the past, relishing the idea that he was literally following in the footsteps of icons such as King.

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In Boston, march organizers asked participants older than 21 to take their places behind students spearheading the struggle.

Even as far away as Spain, young people such as Lucia Smith, 6, received an early introduction to political activism. She marched with her mother, Aiko Smith, near the US Embassy in Madrid. The girl carried a sign saying: "Your right to rifles. My right to life. Choose."

Lucia Smith, 6, gets an early introduction to activism by marching Saturday in Madrid.

On a bus with about 250 students bound for Washington from Pittsburgh, chaperone Justin Cooper said he grew up in a region known for hunting but realizes America's gun culture needs to change.

"Our youth are being confronted with these shootings and all the violence, and I think they're looking at it and saying most people support some kind of change ... but yet our laws don't quite seems to be working with the people," Cooper said. "So the youth of this country said, 'Enough is enough.' The kids are running all this."

People gather Saturday morning for the March for Our Lives event along Pennsylvania Avenue in Washington.

Carol Speaks sat on the bus next to her grandson, whose brother was gunned down in Pittsburgh in 2013. Three years earlier, she had lost her son to gun violence, she said. Both cases remain unsolved.

"It happens so often in our neighborhoods," she said of gun violence. "These guns are so easy to get. ... A lot of times people don't act on things until it touches your doorstep and then it's kind of late."

'This is a movement'

In Washington, Leslie Gunn, a teacher who survived the 2012 mass shooting at Sandy Hook Elementary School in Newtown, Connecticut, said her heart pounded with emotion as she prepared to march. Her mind was on the young students and six adults who died at her school.

"We lost 20 children and 6 adults, 154 bullets in five minutes, and nothing was done," she said.

"We had voices and we advocated ... but if these kids now can make the voice that makes the change, we have to do this. Adults need to get on board with them and follow them because they're speaking the truth."

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In Boston, Marjory Stoneman Douglas High School graduate Leslie Chiu said the march was about all gun violence, not just school shootings.

"This is not just in Parkland," she said. "It is in every community, especially those of color. ... This is not a moment. This is a movement."

In Los Angeles, comedienne Amy Schumer thanked students for taking a stand.

"We know it is hard and we know that they will twist our words and laugh at us, and lie and lie and lie and lie and lie," she said.

Women carry signs at the Los Angeles March for Our Lives event.

The White House , with President Donald Trump at Mar-a-Lago in Florida for the weekend, released a statement in support of the marchers: "We applaud the many courageous young Americans exercising their First Amendment rights today. Keeping our children safe is a top priority" of the President.

Former President Barack Obama also voiced his support, writing on Twitter: "Michelle and I are so inspired by all the young people who made today's marches happen. Keep at it. You're leading us forward. Nothing can stand in the way of millions of voices calling for change."

Michelle and I are so inspired by all the young people who made today's marches happen. Keep at it. You're leading us forward. Nothing can stand in the way of millions of voices calling for change. — Barack Obama (@BarackObama) March 24, 2018

'I might die today'

The mass shooting at Stoneman Douglas, the eighth school shooting of the year, moved the young survivors-turned-activists to push lawmakers to address gun violence in American schools with comprehensive gun control legislation, including bans on assault weapons and high-capacity magazines.

Sophie Phillips holds a sign at a Florida rally for those heading to March for Our Lives in Washington.

"They don't know what it's like to be 20 feet from an AR-15," Parkland survivor Alfonso Calderon , 16, told students at a Washington charter school this week, referring to US lawmakers.

Calderon and other Marjory Stoneman Douglas survivors attended a #NeverAgain -- as their movement is known -- rally this week at Washington's Thurgood Marshall Academy, where two students were killed in separate neighborhood shootings in the last year.

"Every single day you wake up and it might be a thought in your head, 'I might die today,'" Calderon told the students.

"I only had to go through it once. You guys go through it every single day. This isn't a discussion anymore. This is action because that's what we need."