Thousands of people marched Saturday in Philadelphia, Washington, D.C., and other cities across the nation to protest gun violence and demand stronger gun control.

The rallies were part of the March for Our Lives movement, which was spearheaded by survivors of the Valentine's Day shooting at Marjory Stoneman Douglas High School in Parkland, Fla., that killed 17 people.

In Philadelphia, large crowds gathered outside Independence Hall before marching through Old City to Columbus Boulevard, where students and politicians spoke.

Sen. Bob Casey (D., Pa.) told the crowd that assault rifles should be banned. The father of a freshman at the Parkland school told the Philly gathering that he turned in his AR-15 after the shooting.

Another speaker pointed out that black youth have long tried to call attention to gun violence in their neighborhoods.

>> Read more: The world is listening to Parkland teens. Some Philly kids wonder: Why not us?

A mother who lost her son to gun violence decried weapons.

The march began around 10 a.m. and lasted several hours. Here were some of the best signs that people made.

Washington's march began at noon, and huge crowds showed.

Among the Philadelphians in D.C. was George Manigault, 76, who marched for civil rights as a teenager.

Marches also unfolded in Philadelphia's suburbs, including West Chester, Media, and Haddon Heights.

People marched for a variety of reasons: Some had lost loved ones to gun violence, while others wanted to feel safe in schools.

Why are you marching?

In Bucks County, students served detention for a previous protest

At Pennridge High School in Perkasie, more than 200 students defied school orders when they walked out on March 14, the one-month anniversary of the Parkland shooting, to protest gun violence. Some of those students served detention last weekend, while others did so Saturday.

A handful of parents showed up with signs to support their children.

One girl called the detention "a badge of honor."

Across the street from the parents, some seniors at the high school showed up to support gun rights.

To see more photos from Saturday's marches, check out our gallery.