While numerous school districts gave their blessings to the walkouts, others said anyone who took part would face discipline. Many students defied the warnings and left school anyway.

They included over two dozen at Lindenhurst High School on New York state’s Long Island, who were at first suspended, then had their punishment reduced to detentions, according to a senior and the school superintendent.

In Parkland, thousands of students slowly filed onto the Stoneman Douglas school football field to the applause of families and supporters beyond the fences as law enforcement officers looked on. News helicopters hovered overhead.

Ty Thompson, the principal, called for the “biggest group hug,” and the students obliged around the 50-yard line.

“We want change!” students chanted on the pavements outside the school. “Can you hear the children screaming?” read one of the signs.

But not all students in Florida were in favour of gun control. About 80 miles (129km) north of Parkland at Vero Beach High School, chants of “No More Silence, end gun violence,” were countered by shouts of “Trump!” and “We want guns” from other students, according to video posted by local newspaper TCPalm.

The protests continued nationwide. In the nation’s capital, more than 2,000 people - including students from Washington-area schools - gathered outside the White House chanting “Hey, hey, ho, ho. The NRA has got to go!” and “What do we want? Gun control! When do we want it? Now!”

“We don’t feel safe in schools any more,” said Sarah Chatfield, a high school student from Maryland, standing with hundreds of other protesters outside the White House.

Some pupils, many of whom will be able to vote in 2020, also marched to the US Capitol, where Democratic lawmakers emerged from the white-domed landmark to praise them.

US President Donald Trump, whose initial commitment to restricting guns has withered in the face of pressure from gun lobbyists, was travelling in Los Angeles and was not in the White House during the demonstrations.

At New York City’s Fiorello H LaGuardia High School, crowds of students poured into the streets of Manhattan, many dressed in orange, symbolic of the bright colour worn by hunters to avoid being shot by accident.

“Thoughts and prayers are not enough,” read one sign at LaGuardia, a jab at a response often uttered by lawmakers after mass shootings.

In Akron, Ohio, hundreds of students wearing orange T-shirts with black targets on the front walked out of Firestone High School.

At Granada Hills Charter High School in Los Angeles, students laid prone on the field of a football stadium to form a giant #ENOUGH, symbolising the thousands of youth who die of gun violence every year in the United States.

Students at Columbine High, Colorado remembered the 1999 massacre at their school that began an era in which mass shootings became common in US schools.

“I grew up in a community still haunted by the tragedy from 19 years ago,” said 16-year-old sophomore Abigail Orton.

The walkouts were part of a burgeoning, grass roots movement prompted by the Parkland attack and came 10 days before major protests planned in Washington and elsewhere.

Survivors have lobbied lawmakers and Trump in a push for new restrictions on gun ownership, a right protected by the US Constitution’s Second Amendment.

The walkout by students was matched by a 17-minute blackout by Viacom across all its channels, including MTV, Comedy Central, Nickelodeon and BET.

The student-led initiative helped bring about a tightening of Florida’s gun laws last week, when the minimum age of 21 for buying any handguns was extended to all firearms. But lawmakers rejected a ban on the sort of semi-automatic rifle used in the Parkland attack.

In Washington, however, proposals to strengthen the background-check system for gun sales, among other measures, appear to be languishing.

After protests began on Wednesday, the NRA tweeted a picture of a semi-automatic rifle with the caption “I’ll control my own guns, thank you.”

Students from more than 3,000 schools and groups joined the walkouts, many with the backing of their school districts, according to the event’s organisers, who also coordinated the Women’s March protests staged nationwide over the past two years.

In Newtown, Pennsylvania, more than 100 students walked out of Council Rock High School despite warnings they would face discipline if they left the building.

But after the walkout, Superintendent Robert Fraser said “the level of maturity and sincerity was amazing” among protesters, and the school district waived any punishments.

At Norton High School in the rural-suburban district in northeastern Ohio, a small group of students, including a teenage boy with an American flag draped over his shoulder, stood apart from a larger gathering of nearly 300 students who walked out of class. One of the students also flew a large Trump flag at the end of his truck.

Ryan Shanor, the school’s principal, said the small group wanted to honour the victims but disagreed with sentiment they considered to be against the Second Amendment.

“They did not agree with everything they thought the protest was about,” he said.

This article contains additional information from Agence France-Presse