Hundreds of thousands of demonstrators gathered in the nation’s capital and cities across the country Saturday to demand action against gun violence, vividly displaying the strength of the political movement led by survivors of a school massacre in Parkland, Fla.

Organized by students from Marjory Stoneman Douglas High School, where a gunman killed 17 last month, the March for Our Lives showcased impassioned teens calling on Congress to enact stricter gun-control laws to end the nation’s two-decade stretch of campus shootings.

Hundreds of “sibling protests” took place across the world, from New York City — where demonstrators spread across 20 blocks — to Jonesboro, Ark., a small city marking the 20th anniversary of a middle-school shooting that left four students and a teacher dead. Gun-rights advocates mounted counterprotests in Salt Lake City, Boise and Valparaiso, Ind., where one sign read “All Amendments Matter.”

Although the D.C. march was funded by Oprah Winfrey, George and Amal Clooney, and other celebrities, Stoneman Douglas High students have been its faces. Their unequivocal message to legislators: Ignoring the toll of school shootings and everyday gun violence will no longer be tolerated.

“To the leaders, skeptics and cynics who told us to sit down, stay silent and wait your turn: Welcome to the revolution,” Cameron Kasky, a Stoneman Douglas student, said to a crowd that packed at least 10 blocks of Pennsylvania Avenue. “Either represent the people or get out. Stand for us or beware. The voters are coming.”

The main march in Washington was a heady mix of political activism, famous entertainers and the undisguised emotion of teenagers confronting the loss of friends and loved ones in a national spotlight.

Sam Fuentes, a senior shot in the leg at Stoneman Douglas, threw up on stage while delivering her speech to a national television audience. She recovered and led the crowd in a rendition of “Happy Birthday” for her slain classmate, Nicholas Dworet, who would have turned 18 on Saturday.

Emma González, 18, took the stage in a drab olive coat and torn jeans, speaking of the “long, tearful, chaotic hours in scorching afternoon sun” as students waited outside Stoneman Douglas High on the day of the shooting.

With a flinty stare, tears streaming down her face, González stood silent on the rally’s main stage for nearly four minutes — evoking the time it took Parkland shooter Nikolas Cruz to carry out his attack. The crowd began chanting, “Never again.”

The moment was widely shared on social media. “Fight for your lives before it’s someone else’s job,” González said before she left the stage.

The march emphasized not just the highly publicized mass shootings in suburban, white schools, but also the far more common shootings that leave one or two young people dead and often affect predominantly black and Hispanic students in poor neighborhoods.

Zion Kelly, a senior at Thurgood Marshall Academy in Southeast Washington, spoke about his twin brother, Zaire, who was shot and killed by a robber in September. Choking back tears before a rapt crowd, Kelly described the close bond they had shared.

“From the time we were born, we shared everything. I spent time with him every day because we went to the same schools, shared the same friends, and we even shared the same room,” he said. “I’m here to represent the hundreds of thousands of students who live every day in constant paranoia and fear on their way to and from school.”

The march drew a huge crowd, though there were no police estimates of its size. One indication: Metro officials reported there had been about 334,000 trips on the system by 4 p.m. Saturday, compared to 368,000 trips by the same time on the day of President Trump’s inauguration. The Women’s March last year generated 597,000 trips by the same time of day.

Because many of the demonstrators were children, authorities in the nation’s capital said they took extra security precautions. By day’s end, police had reported no violent altercations or other problems, despite a small contingent of counterprotesters decrying efforts to toughen gun laws.

“To be honest, I’m scared to march,” Stoneman Douglas senior Carly Novell said in a Saturday morning tweet, citing the risk that a shooter might terrorize those gathered to protest in Washington. “This is a march against gun violence, and I am scared there will be gun violence on the march. This is just my mind-set living in this country now, but this is why we need to march.”

The march offered a window on a generation that has come of age after the 1999 massacre at Columbine High School in Colorado, which left 13 dead and is considered a milestone in the evolution of modern school shootings.

Nearly 200 people have died from gunfire at school since 1999, and more than 187,000 students attending at least 193 primary or secondary schools have experienced a shooting on campus during school hours, according to a Washington Post analysis. The analysis found that Hispanic students are nearly twice as likely as white students to experience gun violence at school, and black students three times as likely.

The most recent shooting took place Tuesday at Great Mills High School in Southern Maryland, where 16-year-old Jaelynn Willey was killed by her 17-year-old former boyfriend, who also died. About 100 Great Mills students attended the march, which drew people from around the country.

Callie Stone, 18, traveled to Washington from Raleigh, N.C.

“We’ve grown up knowing this could happen to us,” said Stone, 18, walking down Pennsylvania Avenue before the march wearing a denim jacket emblazoned with “Nasty Woman,” a term President Trump used to insult Hillary Clinton during the 2016 election and that progressive women adopted as a moniker.

With Stone was her mother, whom Stone had told the previous day that she wasn’t sure she wanted to raise children in a world where students fear going to school. “But I said, ‘Look at you, at your generation — you all are bringing us hope,’” said Kelly Stone, 54.

One couple at the rally, Rebekah and Chris Sullivan, described how their 4-year-old son already performs “active shooter” drills with his class, sitting quietly as his teacher rattles a locked door from the outside.

Jordin Torres, a junior at Howard High School in Ellicott City, Md., said she helps her instructors check that the blackout paper they’re supposed to draw over classroom windows if a shooter attacks is untorn.

Torres carried a sign: “I have a dream that one day I won’t be scared to go to school.”

Other signs read, “It happened at my school,” “Enough is enough!” and “I survived. My daughter didn’t.”

In Boston, where a sister rally was underway, a group of about 25 counterprotesters gathered in front of the gold-domed Massachusetts statehouse to decry calls for tougher gun laws.

“After a tragedy like this one, everyone looks past the motives of the shooter and immediately focuses on guns,” said Robert Johnson, 21, of New York. “If you run over someone with a car, they don’t blame the car. But if someone is shot, they immediately blame the guns.”

As they have spoken out in the wake of the Parkland shooting, Stoneman Douglas students have endured frequent attacks from opponents of gun control, with some even falsely suggesting they were actors paid by liberal activists.

Houston lawyer and gun-rights activist Collins Iyare Idehen Jr., who uses the pseudonym Colion Noir as a host on NRATV, took to the airwaves ahead of the march to say the students were “getting ready to use your First Amendment to attack everyone else’s Second Amendment” and that “no one would know your names” if the shooting had not occurred.

The White House issued a statement Saturday praising the marchers, despite their calls for tougher gun-control measures than President Trump supports.

“We applaud the many courageous young Americans exercising their First Amendment rights today,” White House spokeswoman Lindsay Walters said in the statement, in which she added that “keeping our children safe is a top priority of the President’s.”

The president himself was in Florida at Trump International Golf Club, about 35 miles from Parkland. The spending bill he signed on Friday includes a provision to tighten the nation’s background-check system and may slightly open the door to restoring federal funding for gun research.

The Parkland students have already had an impact on the debate.

Lawmakers in Florida, a state long renowned as a laboratory for gun deregulation measures, passed its first significant gun-control legislation in 20 years this month in response to pressure from the Stoneman Douglas survivors.

They will stage another nationwide student walkout on April 20, the anniversary of Columbine, said David Hogg, one of the movement’s leaders. And they are planning future marches on every state capitol.

It remains unclear whether they can shame Congress into passing new restrictions on guns. Many expected action after the killing of 20 first-graders at Sandy Hook Elementary School in 2012. But although President Barack Obama wept on television and convened a task force to craft stricter gun controls, nothing happened.

James Barden, 17, was in Washington on Saturday, carrying a photo of his 6-year-old brother, Daniel, who was killed at Sandy Hook. Barden and his family have toiled for five years advocating for stricter gun-control laws.

He said he was heartened by the turnout Saturday. “If this doesn’t do anything,” he said, “I don’t know.” Asked how he felt about the demonstration, he replied, “Hopeful.”

Correction: A previous version of this story misstated the age of James Barden. He is 17, not 16.

Moriah Balingit, Kayla Epstein, Mary Hadar, Luz Lazo, Erin Logan, Justin Wm. Moyer, Antonio Olivo, Dana Priest, Katie Shaver, Rachel Siegel, Ellie Silverman, Kelyn Soong, Shira Stein, Patricia Sullivan and Julie Zauzmer contributed to this report.