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WASHINGTON — Marjory Stoneman Douglas senior Emma Gonzalez stood silent, with tears streaming down her face, on a stage in front of thousands at Saturday's March for Our Lives.

After listing the names of the 17 people killed when a gunman rampaged through her school on Valentine's Day, Gonzalez asked the crowd to fathom how so many could be murdered in only 6 minutes and 20 seconds.

And then she stopped speaking.

Silent minutes ticked by. Then an alarm beeped.

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

She added: "Fight for your lives before it's someone else's job."

In the 39 days since the shooting in Parkland, Florida, Gonzalez and her fellow student survivors have galvanized a nationwide movement for gun reform.

On Saturday, hundreds of thousands of students gathered in the nation's capital and at sister marches across the country and around the world to deliver a powerful, unified message: Enough is enough.

Students from Marjory Stoneman Douglas High School organized and hosted the main event on the mall in Washington with the U.S. Capitol in the background, a stark reminder of why they were demanding change.

"We hereby promise to fix the broken system we've been forced into and create a better world for the generations to come," Cameron Kasky, one of the student organizers, told a cheering crowd filled with teenagers. "Don't worry, we've got this."

The Washington rally featured 20 student speakers, numerous famous musicians and the Stoneman Douglas Drama Club, which performed its original song, “Shine," with the student choir. Organizers estimated that 800,000 people attended the march.

Across the country, there were more than two dozen marches that occurred in large cities, including New York, Miami, Cincinnati, Houston, Atlanta, Baltimore, and Pittsburgh. Rallies were also held around the world in Israel, New Zealand, Australia, the U.K., Japan, Belgium, India, France and Chile.

In total, organizers estimated there were approximately 800 marches organized on Friday and Saturday — even one in Antarctica.

In Washington, rally-goers were chanting “Vote them out! Vote them out!” before the event began. Hundreds of homemade signs were held up above the crowd: "Stop the madness/vote them out.” Another said, “18th century law can’t regulate 21st century weapons.” And another put it bluntly: “What’s worth more? Your children or your guns?”

But Parkland survivor Jaclyn Corin said Saturday's rally was also about recognizing that gun violence is a nationwide problem.

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"We recognize that Parkland received more attention because of its affluence," she said. "But we share this stage today and forever with those communities who have always stared down the barrel of a gun."

Trevon Bosley, 19, talked about the gun violence in Chicago that killed his brother Terrell Bosley on the way to church in April 2006. He said that these gun deaths are tied to the White House and American political leadership.

“When you have a president that would rather constantly talk about and belittle Chicago's violence rather than send funds and resources, you have gun violence,” Bosley said.

“It's time for the nation to realize gun violence is more than just a Chicago problem or Parkland problem but an America problem,” he added.

But it wasn't just recent graduates and high school students who took the stage on Saturday. Naomi Wadler, 11, of Alexandria, Virginia, spoke as well. She led a walkout at her elementary school on March 14.

“It's time for the nation to realize gun violence is more than just a Chicago problem or Parkland problem but an America problem."

“I’m here today to represent the African-American girls [killed by gun violence] who don’t always make the front pages of the newspapers," she said. The crowd roared in response.

"My friends and I might still be 11, we might still be in elementary school, but we know," she added. "We know life isn't equal for everyone and we know what is 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."

The rally also memorialized the 50th anniversary of Martin Luther King Jr.'s assassination, as Christopher Underwood, 11 — who lost his older brother to gun violence in Brooklyn, New York — reminded the crowd that the civil rights icon's death was the result of gun violence.

Those in attendance were surprised when King's granddaughter, Yolanda Renee King, 9, later came to the stage alongside Stoneman Douglas junior class president Jaclyn Corin with a microphone.

"My grandfather had a dream that his four children would not be judged by the color of their skin but by the content of their character," she said. "I have a dream that enough is enough and that this should be a gun-free world. Period."

By the early afternoon, the march in Washington had drawn hundreds of thousands of people from all over the country. Participants arrived hours ahead of the noon start-time on Saturday to get a spot toward the stage, where Miley Cyrus, Lin-Manuel Miranda and Ben Platt and Ariana Grande performed.

Lawmakers from the House and Senate left Washington on Thursday for a two week-recess marking Easter and Passover. Numerous Democratic lawmakers remained for the event, however, including Florida lawmakers Sen. Bill Nelson and Rep. Debbie Wasserman Schultz.

President Trump left on Friday evening for his Mar-a-Lago property in Palm Beach, Florida. A White House spokeswoman released a statement prior to the event.

“We applaud the many courageous young Americans exercising their First Amendment rights today. Keeping our children safe is a top priority of the President's, which is why he urged Congress to pass the Fix NICS and STOP School Violence Acts, and signed them into law," Deputy Press Secretary Lindsay Walters said.

Stoneman Douglas survivor Delaney Tarr said the current legislation offered was not nearly enough.

"We are not here for bread crumbs; we are here for real change," she said. "We are here to lead, we are here to call out every single politician to force them into enacting this legislation."

The power of the moment was not lost on the thousands of students, teachers and parents in attendance.

“To anybody who thinks their voice doesn’t matter, it does,” Samantha Goldberg, a 17-year-old student at Stoneman Douglas, said as she was walking to the rally Saturday. “Every single voice comes together and makes a huge impact. This is just the beginning. This march is the first step in a long process, but ultimately there’s going to be a huge change. I mean, there’s not half a million people here for nothing.”

Michelle Lo Brutto traveled to the march in Manhattan on Saturday with her family and her cousins from their home in Old Bridge, New Jersey. Around her neck she wore a sign demanding gun control in memory of her sister, Cristina.

In 2012, a gunman, armed with an AK-47, opened fire inside the supermarket where he worked with Cristina Lo Brutto. Cristina, 18, and her colleague Bryan Breen, 24, were both shot and killed. The gunman later killed himself.

"He had severe mental health issues and was hospitalized three times and he was still able to buy [a gun] legally," Michelle Lo Brutto said. "I think everyone has just had enough, they're just tired of seeing people they love get hurt or killed and they just don't want to be the next one."

Many in attendance felt they had been impacted by gun violence in one form or another, and said they were tired of the status quo.

Sophie Zipoli, an 18 year-old high school senior from Burlington, Connecticut rallies at the March for Our Lives in Washington, DC on March 24, 2018. Jack Sears

“Sandy Hook happened not too far from my hometown several years ago,” said Sophie Zipoli, a high school senior who traveled to Washington from Burlington, Connecticut. “After that, I thought there’d be some significant change, yet here we are years later and there still so many mass shootings and school violence.”

Chicago native Ke’Shon Newman’s 16-year-old brother was shot and killed a few blocks away from his home in Chicago’s south side. Now, the 15-year-old routinely checks an app on his phone that tracks gun deaths in his city.

Newman told MSNBC that he traveled to Washington Saturday, to not only demand safety for students in America’s classroom, but an end to gun violence in general.

“I just want the world to be a more safe environment for everyone, this march today is going to bring more awareness around the country,” he said. “Let’s just stop all the gun violence in this country and let’s get to action.”

Teachers and parents also attended the rally and they were not shy about sharing their opinions on gun violence.

“It’s unacceptable to me that our children live in fear of something like what’s been going on in our schools,” said Deb Sheridan, an eighth grade teacher from Leesburg, Virginia. “We need to address gun control issues and make our children feel safe in schools — that’s the one place that they should be going in and looking for an education, not worrying about their safety.”

Richard McDonald, 70, came from Detroit to Washington to visit his granddaughter and bring her to the March.

“I know that they’re not fully engaged,” he said of Trump and Congress, “and I think the only way to [create change] is to keep demonstrating that the public is against more guns. The idea of arming teachers is insane and it’s not going to make things any better. What makes things better is not having guns at all.”

Rebecca Shabad reported from Washington. Chelsea Bailey and Phil McCausland reported from New York. Jack Sears contributed reporting from Washington.