Thousands of US students have walked out of classrooms, waving signs and chanting slogans like "we want change" in a coast-to-coast protest against gun violence prompted by a deadly rampage at a Florida high school last month.

The #ENOUGH National School Walkout began at 10am Irish time and was scheduled to last 17 minutes, though many protests lasted longer.

The protest rolled westward, with students in other time zones also walking out, including at Colorado's Columbine High School, where two gunmen killed 13 people in 1999.

The announced duration of the walkouts was intended to commemorate the 17 students and staff killed at Marjory Stoneman Douglas High School in Parkland, Florida, on 14 February.

The massacre was the latest in a series of shootings that have plagued US schools and colleges since the Columbine attack.   

While many school districts gave their blessings for the protests, others warned of discipline for any students who joined the walkout, though many defied the warnings and left school anyway.

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 police looked on. 

Ty Thompson, the school's principal, called for the "biggest group hug," and the students obliged.

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

At New York City's Fiorello H. LaGuardia High School, crowds of students poured into the streets of Manhattan, many dressed in orange, the colour adopted in recent years by the gun-control movement.

"Thoughts and prayers are not enough," read one sign, needling the rote response many politicians make after mass shootings.

Later, hundreds of students sat down on the pavement, filling half a city block, and fell silentt.

The walkouts were part of a burgeoning, grassroots movement that grew out of the Parkland attack.

Some survivors have lobbied state and federal politicians, and even met President Donald Trump, to call for new restrictions on gun ownership, a right protected by the Second Amendment of the US Constitution.

"We don't feel safe in schools anymore," said Sarah Chatfield, a high school student from Maryland, standing in a crowd of hundreds protesting outside the White House, with some sitting silent with their backs turned.

"Trump is talking about arming teachers with guns," the 15-year-old said. "That is not a step in the right direction."           

Some of the students began marching toward Capitol Hill.

"Hey hey, ho ho, the NRA has got to go!" they chanted, referring to the powerful gun-rights interest group, the National Rifle Association.

Some Democratic politicians emerged from the Republican-controlled US Congress to praise the students.

The Parkland survivors' efforts helped bring about a tightening of Florida's gun laws last week, when the minimum age or buying any kind of gun was raised to 21 years from 18, although politicians rejected a ban on the sort of semi automatic rifle used in the Parkland attack.

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

Students crammed into a packed hearing today before the US Senate Judiciary Committee about gun laws and school safety, which was to include testimony from federal law enforcement officials and the father of one of the Parkland victims.

The House of Representatives was set to debate and pass a bill today to spend $50 million a year on training schools and law enforcement agencies to detect potential violent acts before they occur.

It would not, however, allow any of the money to be used to arm teachers or other school officials.

The measure has broad bipartisan support.

Students from more than 2,800 schools and groups are joining the walkouts, many with the backing of their school districts, according to the event's organisers, who also co-ordinated the Women's March protests staged nationwide over the past two years.

The tens of thousands of pupils who walked out of schools to demand action on gun control in one of the biggest student protests the US has seen have won support from Hollywood stars.

Ben Stiller and Amy Schumer were among those to praise the walkouts in the growing movement that emerged in the wake of the Marjory Stoneman Douglas High School massacre.

The protests took place a day after Florida prosecutors said they would seek the death penalty for Nikolas Cruz, who has been charged with 17 counts of premeditated murder and 17 counts of attempted murder in the Parkland attack.

A Florida judge has this evening entered a not-guilty plea for Cruz, who faces a possible death penalty for a massacre at a high school last month that killed 17 people and triggered today's nationwide walkout of students.

The 19-year-old chose to remain silent during a hearing in Broward County circuit court.

Judge Elizabeth Scherer then entered the not-guilty plea on his behalf for 17 counts each of first-degree murder and attempted murder..
Cruz, wearing an orange jail jumpsuit and shackled at wrists and ankles, sat alone and kept his head bowed in what is normally the jury box while guarded by a dozen deputies.

Nine relatives of victims of the attack at Marjorie Stoneman Douglas High School attended the hearing.