Yet the role of leading gun control groups and advocates, and even local organizers, has plainly aggravated and frustrated critics of Saturday’s marches. Breitbart, the conservative website, said the events were “directed, promoted and funded by left-wing adults and adult-led organizations.”

Although adults were involved in some planning — an organizer listed on the National Park Service permit for the march in Washington, for example, is a prominent activist from California — there is little doubt that students were crucial to orchestrating Saturday’s events. In Washington, the area seemed to influence the program, which included Parkland students and celebrity guests but also a choir from Baltimore and remarks from a local 11-year-old girl.

“The kids did everything,” said Jenn Hoadley, 36, who helped students organize a march in Anchorage. “All I did was say, ‘You want a stage? Cool. I’ll find one for you. You need a sound system? Cool. I’ll find one for you. You want to march in the park? I do paperwork to help you get that done.’ They planned it all, and they should be given credit for that.”

In New York, a Facebook group, envisioned to coordinate the efforts of about 30 friends and classmates to attend a march, swelled into the planning apparatus for the city’s major event. The group’s creator, Alex Clavering, a Columbia Law School student, was quickly regarded as the official organizer.

So that was what he became. He convened meetings and established a fund-raising effort. He applied for a city parade permit — at first, he recalled, for a turnout of 3,000 to 5,000 people. By the end of the first week, he had changed it to 30,000 people. (The mayor’s office ultimately estimated that nearly 200,000 people marched on Saturday.)

The early days, Mr. Clavering, 26, said, were “decentralized, figuring it out, being scrappy.” But more established gun control groups soon got in touch, offering to connect him to others and, eventually, to pick up expenses that included lighting, sound equipment and stages.

Mr. Clavering’s GoFundMe campaign raised more than $25,000, but he said the cost for the entire event was likely around $100,000.