Saturday's marches will cover more than 800 locations around the country and outside the USA, in cities including London, Paris, and Tokyo, according to the march website.
At the largest March For Our Lives protest in the U.S. on Saturday, demonstrators jammed Washington's Pennsylvania Avenue, where they listened to speeches from survivors of the Feb 14 mass shooting at Marjory Stoneman Douglas High School in Parkland, Florida.
"If you listen real close, you can hear the people in power shaking", Parkland survivor David Hogg said to roars from protesters packing Pennsylvania Avenue from a stage near the Capitol to a spot near the White House many blocks away.
McCartney said his decision to take part in the New York City rally was prompted by the 1980 Manhattan shooting death of John Lennon, his former Beatles bandmate.
"Let's get to the point were parents don't have to worry about children going off to school and whether they are going to be alright or not", Kovacs said. Organizers on Saturday estimated 800,000. "We are demanding the prohibition of sales of high-capacity magazines and we are demanding universal background checks, which is something you'll see, from the polls, pretty much the entire country is behind and yet we've seen nothing of it", said student organizer Cameron Kasky.
It might be hard for the United States lawmakers and President Donald Trump to ignore this one, if for no other reason than the sheer numbers chanting, "Enough is enough" and demanding legislation to control gun.
"I work extremely hard at my studies".
"I was organizing an ALICE training for mentors when one of my mentors came in and said, 'Your timing couldn't be better because there is a shooting going on in Parkland, Fla'".
"People have said I am too young to have these thoughts on my own", she said.
Showing how social activism can span generations and the parallel between these protests and other historical youth-led movements, Yolanda Reneee King, the granddaughter of civil rights leader Martin Luther King, Jr.is there.
"I have a dream that enough is enough, and that this should be a gun-free world".
Officials say the rally in Washington, D.C. drew more than 300,000 demonstrators.
"I represent the African-American women who are victims of gun violence, who are simply statistics instead of vibrant, lovely girls full of potential", Wadler said.
Among those marching next to New York's Central Park to call for tighter gun controls was pop star Paul McCartney, who said he had a personal stake in the debate.
"I really do believe that", he told CNN, adding that the students' momentum must continue.
That was where the extraordinarily eloquent voice of Emma Gonzalez from Parkland, Florida, one of Marjory Stoneman Douglas High School's survivors and leaders spoke: "Everyone who was there understands". Estimates from two other sources, however, give wildly different ideas of how many people actually attended.
The Trump administration also took a step on Friday to ban the sale of bump stocks - devices that enable semi-automatic weapons to fire like machine guns - that helped gunman Stephen Paddock massacre 58 people in Las Vegas in October.
Wadler and her friend led a walkout at their school, George Mason Elementary in Alexandria Virginia, on March 14, just a week and a half before she headed to D.C. where over 200,000 people gathered.
Carrying signs and chanting "vote them out" and "enough is enough", the crowd marched from city hall to the Alamo.
The president's call to arm certain teachers fell flat at the protest, and among critics as young as Zoe Tate, 11, from Gaithersburg, Maryland.
"Tell me if I'm insane but how is it that it's 2018 and I go to school and I'm [more] anxious about what I do if there's an active shooter than my geometry test?" another student asked.