As millions of Americans participated in the March For Our Lives in cities across the United States on Saturday, sports figures took to the streets and social media to lend their voices to the cause.

Among them was Chicago Cubs first baseman Anthony Rizzo, a 2007 graduate of Marjory Stoneman Douglas High School, where 17 students and staffers were killed by a former student armed with an AR-15 rifle on Feb. 14 in Parkland, Florida.

Rizzo posted a photo of his family members marching "for their lives" in Florida. He said he was inspired by the students at Stoneman Douglas and every student, parent and teacher across the country who participated in the march.

My family marching for their lives. I am so proud to be from Parkland and so inspired by not only the students at MSD but every student, parent and teacher across the country who participated. #MarchForOurLives pic.twitter.com/pzZ82ozV3Z — Anthony Rizzo (@ARizzo44) March 24, 2018

Survivors of the high school massacre launched a movement to try to persuade lawmakers to address the issue of gun violence and school shootings. Marches took place all over the country, with an estimated 800,000 marchers participating in the largest one, in Washington, D.C.

Protesters are demanding new laws and programs that they believe will curtail mass shootings at schools and elsewhere.

Many sports figures participated in the march or urged others to do so, including Los Angeles Lakers owner Jeanie Buss, Golden State Warriors coach Steve Kerr, Basketball Hall of Famer Bill Russell and former two-time NBA MVP Steve Nash. Russell was in Washington, D.C., in 1963 for a civil rights march that included the Rev. Martin Luther King Jr.'s "I Have a Dream" speech.

March For Our Lives Manhattan Beach Pier @sleek08 @kwaquadeep pic.twitter.com/Z00vUdQ7wp — Jeanie Buss (@JeanieBuss) March 24, 2018

Reminder today to get out and march! Thank you to the young generation for inspiring all of us and reminding us that change will only happen through our own will. Register to vote and demand common sense gun laws from any and all potential elected officials! Democracy will win! — Steve Kerr (@SteveKerr) March 24, 2018

If you can get out & support @AMarch4OurLives today I am proud of this generation standing up for injustice & having their voices heard, making real change. The world is listening. Some may not know me but I stand with you @Emma4Change #marchforourlives @MSNBC @mtv @naacp @NBA pic.twitter.com/ypSeeT1LVH — TheBillRussell (@RealBillRussell) March 24, 2018

I'm marching in LA today. Please find your nearest march and support this life and death matter. #NeverAgain https://t.co/CFMviwgUPu — Steve Nash (@SteveNash) March 24, 2018

Philadelphia 76ers rookie Ben Simmons warmed up for Saturday night's game and was introduced while wearing a blue March For Our Lives T-shirt.

"For me growing up in Australia, I think there was a gun ban way back in the day where they bought back the guns and there hasn't been any major shootings and no one carries around guns," Simmons said. "You want kids to feel safe going to school."

Others, including Miami Heat guard Dwyane Wade, who, with his wife, Gabrielle Union, donated $200,000 to help send kids to the march, used social media to highlight their messages. Oklahoma City Thunder forward Carmelo Anthony helped pay to send more than 4,500 kids from Baltimore to the march in Washington, D.C.

#MarchForOurLives Salute the youth making a REAL impact!! ❤️❤️ — LeBron James (@KingJames) March 24, 2018

Humbled and proud of this generation taking the lead. Their time is now. #MarchForOurLives pic.twitter.com/6WpG7AxMKK — Chris Paul (@CP3) March 24, 2018

New England Patriots owner Robert Kraft sent the team's plane to fly families of the Parkland victims and several other students who were injured in the attack to Washington, D.C.

In Parkland, Saturday's march approaching Stoneman Douglas snaked for two miles, with thousands of students, teachers, parents and supporters chanting in favor of tighter gun laws that they believe would have prevented last month's massacre.

"Enough is enough," they shouted. "No more AR-15s," referring to the semi-automatic rifle the killer used. But when they reached the school, the March For Our Lives participants went silent to honor the 17 students and staff members who died.

Marchers took to the streets in other cities such as Boston, New York, Chicago, Dallas, Houston, Minneapolis, Seattle, Phoenix, Denver, Los Angeles and Oakland, California.

The Associated Press contributed to this report.