Summoned to action by student survivors of the Florida school shooting, hundreds of thousands of teenagers and their supporters rallied in the U.S. capital and cities across the U.S., Canada and the globe on Saturday to press for gun control in one of the biggest youth protests since the Vietnam War era.

"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."

Cameron Kasky, who survived the Feb. 14 school shooting at Marjory Stoneman Douglas High School in Parkland, Fla., gives impassioned speech at March for Our Lives rally in D.C. 3:58

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.

U.S. 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.

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, Fla., the site of the Feb. 14 attack at Marjory Stoneman Douglas High School that left 17 people dead.

Protests outside U.S.

More than 800 demonstrations were scheduled worldwide, with U.S. protests set from San Clemente, Calif., to New York and Parkland, according to gun-control group Everytown For Gun Safety.

At least eight rallies were planned in Canadian cities, from Victoria to St. John's. By midday, about 200 people had gathered in downtown Toronto to show their support for the cause.

Paul McCartney on CNN: “One of my best friends was killed in gun violence right around here, so it’s important to me." <a href="https://t.co/yloH9QWMgC">pic.twitter.com/yloH9QWMgC</a> —@Mr_Berman

Day 6 | Students make their voices heard on gun violence

Cyril Yared, who graduated from Stoneman Douglas last year, said he planned to attend a protest in Montreal, where he now attends McGill University.

"Really this movement is a result of the passion of young people who said that we have had enough and that we need change," he told CBC News. "So as long as these young people remain passionate, as long as they persist, even if we don't get the progress that we want immediately, I think that we're going to see change now and in the long-term."

Protesters Daisy Hernandez of Virginia, right, and Hunter Nguyen of Maryland hold their hands up as they wait for the beginning of the March for Our Lives rally on Saturday in Washington. (Alex Wong/Getty Images)

Gun-rights supporters

Pro-gun activists held counter-protests at some cities across the U.S. on Saturday.

In Salt Lake City, about 500 pro-gun marchers walked to the state capitol building, advocating for fortified schools and more armed teachers. An hour later, about 6,000 anti-gun violence demonstrators marched the same route in a call for more gun regulations.

About two dozen gun-rights supporters staged a counter-protest in Phoenix, holding flags and sometimes challenging opponents to debate gun issues.

They were far outnumbered, however — the Arizona Department of Public Safety estimates that 15,000 people attended the March for Our Lives gun-control rally at the state capitol. Two of the student organizers of the event opened it by urging young people to register to vote and boot out officeholders who won't act.

In Washington, about 30 gun-rights supporters staged a counter-demonstration in front of FBI headquarters, 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 to 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.

‘I am a teacher not a sniper.’ A sense of today’s march. <a href="https://t.co/423m6xYPT0">pic.twitter.com/423m6xYPT0</a> —@EMauroCBC

'I have a dream that enough is enough'

The Rev. Martin Luther King Jr.'s granddaughter Yolanda Renee King, 9, 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 Florida, the police presence was heavy as more than 20,000 people filled a park near Stoneman Douglas 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."

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."

Cyril Yared talks to CBC News about the student-led protests 1:04

Powerful opposition

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 counterpoint from the National Rifle Association 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.

The protesters, many of them high school students, claim that the youth leadership of this initiative is what will set it apart from previous attempts to enact stronger gun-control legislation.

A boy holds a sign while rallying in the street during a March for Our Lives event at the Miami Beach Senior High School in Miami on Saturday. (Javier Galeano/Reuters)

In Atlanta, Lindsey Alexander, a freshman at Decatur High School in Decatur, Georgia, attended her first protest, inspired by hearing Parkland students debate the NRA on television.

"If nothing changes, we're going to continue to have school shootings," she said. "I understand the Second Amendment is important. We've always had this right. But when the Founding Fathers put that right in place, they didn't mean it to become what it is today."

Polls indicate that public opinion nationwide may be shifting on an issue that has simmered for generations, and through dozens of mass shootings.

A new poll conducted by The Associated Press-NORC Center for Public Affairs Research found that 69 per cent of Americans think gun laws in the U.S. should be tightened. That's up from 61 per cent in 2016 and 55 per cent in 2013.

Overall, 90 per cent of Democrats, 50 per cent of Republicans and 54 per cent of gun owners now favour stricter gun laws. At the same time, the poll found that nearly half of Americans do not expect elected officials to take action.

Among the questions facing march organizers and participants will be how to translate this one-day event into legislative change.

One way they hope to do that is by registering young voters and channeling energy into the midterm elections.