(Adds counterprotest in Florida, background on orange color of movement, details on protester demands)
PARKLAND, Fla., March 14 (Reuters) - U.S. students spilled out of classrooms by the thousands on Wednesday, chanting slogans like "No more silence" and "We want change" as part of a coast-to-coast protest over gun violence prompted by last month's massacre at a Florida high school.
The #ENOUGH National School Walkout was intended to pressure federal and state lawmakers to tighten laws on gun ownership despite opposition by the National Rifle Association (NRA), the powerful gun rights advocacy group.
With some students dressed in orange, the color adopted by the gun control movement, the walkouts began at 10 a.m. local time in each time zone and were scheduled to last 17 minutes, though many rallies went longer.
The duration was a tribute to the number of students and staff killed at Marjory Stoneman Douglas High School in Parkland, Florida, on Feb. 14. It was the latest in a series of shootings that have plagued U.S. schools and colleges over the past two decades.
While many school districts gave their blessings to the walkouts, others said anyone who participated would face discipline. Many students 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 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 sidewalks outside the school. "Can you hear the children screaming?" read one of the signs.
But not all students in Florida were in favor of more gun control. Around 80 miles (129 kms) 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.
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 color 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.
Hundreds of students wearing orange t-shirts with black targets on the front walked out of Firestone High School in Akron, Ohio. One student used a bullhorn and recited an anti-violence poem as parents stood on the sidewalks in solidarity.
At Granada Hills Charter High School in Los Angeles, students laid prone on the field of a football stadium to form a giant #ENOUGH, symbolizing the thousands of youth who die of gun violence every year in the United States.
The walkouts were part of a burgeoning, grassroots movement prompted by the Parkland attack. Survivors have lobbied lawmakers, and even talked with President Donald Trump, in a push for new restrictions on gun ownership, a right protected by the U.S. Constitution's Second Amendment.
"We don't feel safe in schools anymore," said Sarah Chatfield, a high school student from Maryland, standing with hundreds of other protesters outside the White House.
Chanting "Hey hey, ho ho, the NRA has got to go!" some of the students marched to the U.S. Capitol, where Democratic lawmakers emerged from the white-domed landmark to praise them.
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 semiautomatic 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.
SCHOOLS VARY IN RESPONSE
Students from more than 2,800 schools and groups joined the walkouts, many with the backing of their school districts, according to the event's organizers, who also coordinated the Women's March protests staged nationwide over the past two years.
In districts where school authorities warned against joining the demonstrations, some students protested anyway.
More than 100 students walked out of the Council Rock High School North building in Newtown, Pennsylvania, despite warnings from school administration that doing so would bring discipline.
"Students deserve the right to go to school feeling safe and comfortable, not feeling scared that their school will be the next target," a student said into a megaphone to the group outside.
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 honor 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.
(Additional reporting by Gina Cherelus in New York; Jonathan Allen and Alice Popovici in New York; Suzanne Barlyn in Newtown, Pennsylvania; Joe Skipper in Parkland, Florida; Scott Malone in Boston; Kim Palmer in Cleveland; and Susan Heavey, Richard Cowan, Sarah N. Lynch and Ian Simpson in Washington; Lindsey Wasson in Seattle; Writing by Jonathan Allen and Andrew Hay, Editing by Frank McGurty and Jonathan Oatis)