By Keith Coffman

LITTLETON, Colo. (Reuters) – Thousands of students across the United States will mark the 19th anniversary of the massacre at Columbine High School on Friday by walking out of classes, in a show of unity intended to put pressure on politicians to enact tighter gun restrictions.

Students from more than 2,600 schools and institutions are expected to walk out of class at 10:00 a.m. local time, organizers say. They have been asked to wear orange, a color that has come to represent the movement against gun violence, and to observe a 13-second silence in honor of the victims killed at Columbine.

“This movement is here to stay. No more excuses. We want solutions,” organizers said on Thursday on Facebook.

On April 20, 1999, two Columbine seniors killed 12 of their classmates and a teacher before committing suicide. Since then, mass shootings have occurred with shocking frequency across the United States.

The second deadliest public school shooting in U.S. history took place in Parkland, Florida, on Feb. 14, leaving 17 dead. The shooting set off a national student movement calling for an end to gun violence and tighter gun restrictions.

“We can end the daily bloodshed in our country, and we can make history while doing it,” an organizer of Friday’s walkout, Max Cumming, wrote in an open letter to young Americans.

“We can rise up together and declare, with one ringing voice, that the age of national indifference towards the ever-growing death toll is over. We can change America forever, all before we reach 20 years of age.”

Cumming is a senior at Ridgefield High School in Connecticut, near Sandy Hook Elementary School in Newtown, where a mass shooting in December 2012 killed 26 young children and adults.

The walkouts, speeches and drive to sign up voters on Friday are aimed at pressuring U.S. politicians to enact tighter restrictions on gun sales in the run-up to November’s mid-term congressional elections.

On Thursday, Colorado gun control activists rallied near Columbine High School, calling for an end to gun violence.

Columbine has not held classes on April 20 since the massacre, a district spokeswoman said, so there would be no walkout at the school. Students were encouraged to take part in community service.

The latest national rally comes more than a month after tens of thousands of students from some 3,000 schools participated in the #ENOUGH National School Walkout to demand that lawmakers seek tighter gun control regulations.

It also follows “March For Our Lives” rallies in cities across the United States on March 24 that were some of the biggest U.S. youth demonstrations in decades, with hundreds of thousands of young Americans and their supporters taking to the streets to demand tighter gun laws.

Dudley Brown, president of the Colorado-based National Association for Gun Rights, said the gun-control movement seeks to have the government take away rights.

“The main objective of these students is to ban firearms completely, and confiscate the firearms of law-abiding Americans,” Brown said. “We will oppose them at every step.”

(Editing by Catherine Evans and Bernadette Baum)