Students across the US are walking out of their schools to demand action on gun violence following last month's massacre in Florida.
More than 3,000 walkouts were planned across the US and around the world, organisers said.
Students were urged to leave class at 10am local time for 17 minutes - one minute for each victim in the February 14 attack at Marjory Stoneman Douglas High School.
Thousands of students gathered on Pennsylvania Avenue in front of the White House, holding signs and cheering in support of gun control.
The students chanted, "Hey, hey, ho, ho. The NRA has got to go!" and "What do we want? Gun control! When do we want it? Now!"
Remember why we are walking out. We are walking out for my friends that passed, all children that have been taken because of gun violence. We are walking out for the empty desks in my classes, and the unsaid goodbyes. This epidemic of School shootings must stop. #Enoughwalkout— Lauren Hogg (@lauren_hoggs) March 14, 2018
President Donald Trump was in Los Angeles at the time.
Stoneman Douglas High senior David Hogg livestreamed the walkout at the tragedy-stricken school in Parkland, Florida, on his YouTube channel.
Walking amid a mass of people making their way onto the football field, he criticised politicians for not taking more action to protect students.
"By myself, I don't think I have the power. But together with all these people here, I think we can make a change."— NBC News (@NBCNews) March 14, 2018
12-year-old student speaks with @MSNBC's @marianaatencio on #NationalWalkoutDay pic.twitter.com/royw85TJVk
He said the students could not be expected to remain in class when there was work to do to prevent gun violence.
"Every one of these individuals could have died that day. I could have died that day," he said.
From Florida to New York, students poured out of their schools, marching through the streets or gathering on campus to demonstrate.
WATCH: In Washington, D.C., studetns turn their backs on the White House and hold 17 mins. of silence in honor of the students killed at Marjory Stoneman Douglas High#NationalWalkoutDay pic.twitter.com/wToZRDLyHw— MSNBC (@MSNBC) March 14, 2018
Some schools applauded students for taking a stand or at least tolerated the walkouts, while others threatened discipline.
The co-ordinated walkout was organised by Empower, the youth wing of the Women's March, which brought thousands to Washington last year.
Although the group wanted students to shape protests on their own, it also offered them a list of demands for politicians, including a ban on assault weapons and mandatory background checks for all gun sales.
"Our elected officials must do more than tweet thoughts and prayers in response to this violence," the organisation said on its website.
Other protests planned in coming weeks include the March for Our Lives rally for school safety, which is expected to draw hundreds of thousands to the nation's capital on March 24.
Another round of school walkouts is planned for April 20, the 19th anniversary of the Columbine High shooting in Colorado.
Some students in Massachusetts said that after Wednesday's protest, they planned to rally outside the Springfield headquarters of the gun maker Smith & Wesson.
The walkouts drew support from companies including media conglomerate Viacom, which planned to pause programming on MTV, BET and all its other networks for 17 minutes during the walkouts.
Districts in Sayreville, New Jersey, and Maryland's Harford County drew criticism this week when they said students could face punishment for leaving class.
In suburban Atlanta, one of Georgia's largest school systems announced that students who participated might face unspecified consequences. Some vowed to walk out anyway.
"Change never happens without backlash," said Kara Litwin, a senior at Pope High School in Cobb County.
The possibility of being suspended "is overwhelming, and I understand that it's scary for a lot of students," said Lian Kleinman, a junior at Pope High.
"For me personally, this is something I believe in. This is something I will go to the ends of the Earth for."
Other schools sought a middle ground, offering "teach-ins" or group discussions on gun violence.
Meanwhile, free speech advocates geared up for a battle, with the American Civil Liberties Union issuing advice for students who walk out.
Students have turned mourning into a movement. I’m Standing in solidarity with students in Elizabeth, Bellevue, Cherry Hill and across New Jersey and the whole country for #NationalWalkoutDay. We must act to end gun violence. pic.twitter.com/1UUlooLGjI— Senator Bob Menendez (@SenatorMenendez) March 14, 2018
It said schools cannot legally punish them more harshly because of the political nature of their message.
In Pennsylvania, Wisconsin and Texas, some lawyers said they would provide free legal help to students who are punished.
It's only March and nearly 3,000 people have already died from gun violence this year. Gun profiteers and their allies in Congress continue to block any attempt to address this crisis, so our young people are acting. #NationalWalkoutDay pic.twitter.com/QLZiOxDQm9— Keith Ellison (@keithellison) March 14, 2018