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

As hundreds of thousands of Americans participated in the March For Our Lives in cities across the United States on Saturday, sports figures took to the streets and to 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 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 MSD and every student, parent and teacher across the country who participated.

Survivors of the 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 or urged others to march, including Los Angeles Lakers owner Jeanie Buss, Golden State Warriors coach Steve Kerr, NBA Hall of Famer Bill Russell and two-time NBA MVP Steve Nash.

Others, including Miami Heat guard Dwyane Wade, who with his wife, Gabrielle Union, donated $200,000 to send kids to the march, used social media to send 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, the march approaching Marjory Stoneman Douglas High School snaked for two miles Saturday, 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 there.

"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 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.