An estimated 100,000 people are expected to gather in Washington, D.C., on Friday for the 45th March for Life, with President Trump reportedly sending a message of support via video.

The event is among the most high-profile events organized by the nation's opponents of abortion, which was made legal across the U.S. by the Supreme Court's Roe v. Wade decision in 1973.

“Here we are at the 45th year – we never anticipated that we would be here for so long," Jeanne Mancini, president of the March for Life, told Fox News ahead of the rally. "Today, we grieve the loss of life. But it’s also an enthusiastic time; we’re changing hearts and minds."

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Conservative commentator and author Ben Shapiro will be the event's headline speaker, according to the Catholic News Agency. A bipartisan group of lawmakers also will address the crowd. Participants will include Sen. Steve Daines, R-Mont., and Reps. Dan Lipinski, D-Ill., and Chris Smith, R-N.J., plus Louisiana state Rep. Katrina Jackson, a Democrat.

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The march was founded in 1974 by Nellie Gray, an activist and Roman Catholic who died in 2012 at age 88.