On the scene in Parkland, Florida, where "March for Our Lives" was born

PARKLAND, Fla. -- In the 5 1/2 weeks since the massacre in Parkland, Florida, the city has become Ground Zero for gun control activism. An estimated 20,000 people marched through Parkland to the place where, for many, the movement began: Marjory Stoneman Douglas High School.

"As soon as after it happened, and we all got out of the building, it did start a huge movement and change," said Emily Quijano.

The very day the shooting happened, she thought they needed to do something. 

For survivors like her, it was important to stay in Parkland for the march -- close to the families and friends of the 17 who were killed and they now try to honor. 

"A lot of people underestimate us and think we're not as powerful," Quijano said. "But this just shows you so many people have listened to us and so many people can support us and we're not just kids."

Around the country, in hundreds of cities, thousands more marched. In Philadelphia, they came out early.

For some protesters in midtown Manhattan in New York, fear of another mass shooting weighed heavily.

Students at Marjory Stoneman Douglas High School were also present in New York City. Sam Hendler read the names of the victims of last month's shooting.

Their overriding message: A need to change the nation's gun laws. In Indianapolis, demonstrators waited in the snow outside the Statehouse to make that point, like student Isabella Fallahi.

"Right now in indiana it is legal for private citizens to sell their guns with out mandatory background checks," Fallahi said at the rally.

Organizers hope the impact of the marches will carry into upcoming elections -- there was a voter registration area in Parkland and one speaker urged voters to encourage 17 people they know to vote, in honor of the 17 victims.