Organizers say millions of people participated in "No Kings" protests across the nation on Saturday — but how did these events come to be?

American Opposition, which is based in Colorado, is one of the main organizations behind many recent nationwide protests, including the "No Kings" demonstrations protesting the Trump administration.

Carlos Álvarez-Aranyos founded the group and most recently helped lead this weekend's march in Los Angeles.

"I had never seen anything like it in that city, and to lead them in that march was probably the greatest honor of my life," Álvarez-Aranyos said.

Most of Álvarez-Aranyos' work begins in Boulder, Colorado, where he founded American Opposition in January. The group's call to action on its website is to "stand against" fascism, but the messages from participants at each protest vary.

"We know that the country is in existential trouble, and we're fighting to defend it, and ultimately to save it, because it's our country. It's not a politician's country," Álvarez-Aranyos said.

Álvarez-Aranyos explained that American Opposition works alongside protest organization groups like the 50501 Movement and Indivisible to coordinate their efforts in planning and tracking protests in cities across the country.

He also says participation has more than doubled since one of their previously organized national protests in April.

But even as his work centers around calling for national change, Álvarez-Aranyos stresses that he still supports American values.

"I come at this from the immigrants' appreciation of how great this country is, how lucky we are. I think one of the things that's happened is that the American people became complacent based on our prosperity, and now we're remembering that this work, this protection, this democracy thing is a participatory event. We have to show up," Álvarez-Aranyos said.

As protest participation grows, Álvarez-Aranyos says organizing has become a full-time volunteer effort. And with a national spotlight — including a  in April — Álvarez-Aranyos says that safety and security have also become a personal concern.

American Opposition has received almost $13,000 in donations since its start in January, data from the Federal Election Commission shows. One was for $1,000, two were for $500 each, and the remaining $10,979 is unitemized in the federal database.

Álvarez-Aranyos told CBS News Colorado that they only fundraise on their website and ActBlue and have never received a donation larger than $2,000. He said they don't have any large donors but joked that he'd love one.

This weekend's protests were largely peaceful. In Denver, organizer Jennifer Bradley with Colorado's chapter of the 50501 Movement estimates close to 20,000 people were in attendance. The group's name, 50501, stands for 50 states, 50 protests, and one movement. The group also came up with the phrase "No Kings."

The main issues and slogans written by protesters on signs and flags are also varied, but Bradley said that in Colorado, a focus of Denver participants was immigration issues.

"There's a lot of eyes on us, because Colorado is so unique that we won't cooperate with ICE, unlike other states, and so unique that the people are rising up immediately to speak for the immigrant community, because we know how important they are to what makes Colorado what it is," Bradley said.

Both Bradley and Álvarez-Aranyos described their protest experience during the day as joyful.

"I have the chance to stand up for somebody. And that is, to me, one of the biggest gifts, is to be able to lift someone up," Bradley said.

However, there were still late-night clashes with police after the official march had ended. Denver police report six separate marches continued through the evening, leading to arrests.

Officers used loudspeakers announcing that the protesters couldn't enter the highway, video from CBS Colorado's crews and witnesses shows, but DPD said protesters attempted to physically push through the police line. Police declared an unlawful assembly and deployed pepper balls and smoke to try and disperse the crowd.

Bradley said that the local chapter trains volunteers on de-escalation tactics and brings safety leaders and medics to events in an effort to maintain peace. But she acknowledged that the actions of a crowd, especially after the planned event ends, can be difficult to control.

Still, plans for future protests are already in the works. Bradley and Álvarez-Aranyos said they are already looking at another nationwide protest, tentatively for July 4. Álvarez-Aranyos explained that the group plans to continue choosing significant American political holidays as protests continue.

"All of us, coming together. And like that is the — that is the American Dream, protecting the American Dream. And how cool is that?" Álvarez-Aranyos said.

President Trump did not address the weekend protests during the military parade he attended in Washington on Saturday to mark the Army's 250th anniversary.