Students and school staff across the US are commemorating the Florida school shootings with a walkout, exactly one month after the killings.
They are stopping lessons for 17 minutes in memory of the 17 people killed at Marjory Stoneman Douglas High School.
Pupils at the school, which was targeted by a former student, hugged each other on the football field.
Protest organisers accuse Congress of failing to tackle gun violence.
The White House revealed a plan this week to deter school shootings which does not include President Donald Trump's repeated calls to raise the age for buying semi-automatic rifles to 21.
Instead, it moves ahead with his controversial proposal to provide firearms training to school employees.
Tales of heroism from Florida attack
Analysis: One shooting, two Americas
How is the protest unfolding?
The walkouts began at 10:00 in the eastern US (14:00 GMT) and moved west across America's time zones.
Organisers of the National School Walkout, who were also behind the Women's March in January 2017 against Mr Trump's inauguration, called on "students, teachers, school administrators, parents and allies" to take part.
On their website, they accuse Congress of "inaction to do more than tweet thoughts and prayers in response to the gun violence plaguing" schools and neighbourhoods.
Schools taking part included Colorado's Columbine High School where 13 people were shot dead by two students in 1999.
In Parkland, families and supporters applauded as thousands of students slowly marched on to the Stoneman Douglas school football field.
School principal Ty Thompson called on them to stage the "biggest group hug".
A large crowd of students from the Washington area gathered outside the White House holding signs reading "Protect People Not Guns" and chanting "Never again" and "Enough is enough".
Some students also gathered at Capitol Hill where they were addressed by the Senate and House Democratic leaders, Chuck Schumer and Nancy Pelosi.
"We're all moved by your eloquence and your fearless insistence on action to prevent gun violence," Ms Pelosi told them.
"Thank you for bringing your urgency to this fight, to the doorstep of America, the doorstep of the Capitol of the United States."
Students in Cherry Hill, New Jersey, stood in a heart formation to pay tribute to the Parkland victims.
In New York, hundreds of students from Fiorello H LaGuardia High School - many dressed in orange, the colour of the gun-control movement - took to the streets of Manhattan.
"Thoughts and prayers are not enough," read one sign.
New York Governor Andrew Cuomo also joined protesters for a symbolic "die-in" - lying down in a street in Lower Manhattan.
Skip Twitter post by @peterhaskell880
Gov Cuomo joins students in die-in after they walked out of school in Lower Manhattan. Students chant "enough is enough" about gun violence and school shootings. @wcbs880 pic.twitter.com/Wj7jn2lbzJ
— Peter Haskell (@peterhaskell880) March 14, 2018
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End of Twitter post by @peterhaskell880
The disruption to the school day is opposed by some schools, notably in one Texas district where students who walk out have been told they face a three-day suspension.
"We will discipline no matter if it is one, 50, or 500 students involved," said Needville schools superintendent Curtis Rhode.
What happened in Parkland?
The attack on Valentine's Day, 14 February, was the deadliest US school shooting since 2012.
The former pupil arrived on campus and began shooting students and staff before abandoning the weapon and escaping, according to court documents.
Fourteen students and three members of staff died.
US prosecutors are seeking the death penalty for the attacker who has been charged with 17 counts of premeditated murder and 17 counts of attempted murder.
Who are the victims?
How the school attack happened
How have the authorities responded?
The US House of Representatives is due to vote this week on a bill to fund greater security in schools but it does not address gun control.
In its action plan, the White House proposed to:
Fund programmes to train school staff to use firearms
Encourage military veterans and retired police officers to become teachers
Improve background and mental health checks
In addition, a new federal commission on school safety will examine the age limit issue.
Chuck Schumer, leader of the opposition Democrats in the Senate, dismissed the White House's action plan as "baby steps".
When the state of Florida passed a gun control law which raises the legal age for buying rifles to 21, it was sued by America's main gun lobby, the National Rifle Association, which argues that such curbs violate the US constitution.
In a separate incident on Tuesday, a teacher in California is believed to have accidentally fired a gun in a classroom, injuring a student.
Police said the teacher at Seaside High School, in Monterey County, had been taking a public safety class when he accidentally discharged the weapon into the ceiling.
Officials said one student was injured by either bullet fragments or ceiling debris and was taken to hospital for treatment.
The teacher has been placed on administrative leave and the incident is being investigated, police said.
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