Manuel Oliver, whose son, 17-year-old, Joaquin was killed in Parkland, said in Washington, D.C., "Our elected officials betrayed us and have avoided the responsibility to end gun violence."

He said, "If lawmakers who have the power to keep us safe from gun violence are going to avoid taking action," then he's calling for a nationwide strike of schools, from elementary to college.

"Avoid attending school if your leaders fail … to keep us safe," he said. "Avoid going back to school if President Biden fails to open a White House Office of Gun Violence Prevention so that we can finally give this issue the attention that it deserves."

Oliver appeared on stage with David Hogg, a Parkland survivor and March For Our Lives co-founder, who vowed, "This time is different."

"This is not a political issue -- this is a moral issue," Hogg said.

He suggested combatting gun violence the way the U.S. addressed cigarettes.

"With cigarettes, we didn’t just change the laws -- we addressed why people want to smoke in the first place," Hogg said. "We have to address how people get guns and why they feel the need to pick them up in the first place. We must address the fact that the reason why communities like Parkland don't have shootings on a daily basis isn't because we necessarily have the strongest laws … we have some of the most resources."

New York Attorney General Letitia James joined a march in Brooklyn, tweeting, "We will fight every single day until we get the common-sense gun reforms this nation needs to end gun violence and save lives."

Washington, D.C., Mayor Muriel Bowser told D.C. protesters she's frustrated because "we have been here before."

"We're not asking for a lot. We're asking to drop off our children at school without having to worry that someone will bring an AR-15 into their classroom. We are asking to go to the grocery store without worrying that someone will be shot dead by a gunman who is filled with hate. We are asking to let our children go to the playground without worrying that a car will drive by, firing a high-capacity magazine," the mayor said. "We're done asking. We're demanding change and we're demanding change now."

She urged Americans "who share our values to let their senators know that they neither need to make change now-- or get out of our way."

Garnell Whitfield Jr., son of 86-year-old Buffalo, New York, mass shooting victim Ruth Whitfield, told the Washington, D.C., crowd, "We were being naïve to think that it couldn’t happen to us. And until it happened to us, we were sitting on the sidelines."

"Guns by themselves are only one aspect of a much more insidious problem in America," he said, calling out the systems he said radicalize mass gunmen, "filling them with weapons and hate-fueled rhetoric."

"Through their inaction they're giving their tacit approval," he said, demanding the passage of an anti-white supremacy hate crime bill.

The Rev. Denise Walden-Glenn, whose brother died of gun violence in Buffalo, addressed the crowd ahead of Whitfield.

She said she's "working tirelessly to figure out long-term, sustainable solutions" to address gun violence and issues that plague Black and Brown communities across the U.S.

"We need a national government that understands equity," she said. "We are tired of them not valuing us."

She added, "If they don’t give us what we ask for, we will vote them out."