Anthony Rizzo, other athletes react to, participate in March For Our Lives

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.

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.

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.

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.