It looked like a typical Sunday morning at any mega-church. Hundreds packed in for more than an hour of rousing music, an inspirational sermon, a reading and some quiet reflection. The only thing missing was God.

Dozens of gatherings dubbed "atheist mega-churches" by supporters and detractors are springing up around the U.S. after finding success in Great Britain earlier this year. The movement fueled by social media and spearheaded by two prominent British comedians is no joke.

On Sunday, the inaugural Sunday Assembly in Los Angeles attracted more than 400 attendees, all bound by their belief in non-belief. Similar gatherings in San Diego, Nashville, New York and other U.S. cities have drawn hundreds of atheists seeking the camaraderie of a congregation without religion or ritual.

The founders, British duo Sanderson Jones and Pippa Evans, are currently on a tongue-in-cheek "40 Dates, 40 Nights" tour around the U.S. and Australia to drum up donations and help launch dozens of Sunday Assemblies. They hope to raise more than $800,000 that will help atheists launch their pop-up congregations around the world.

They don't bash believers but want to find a new way to meet likeminded people, engage in the community and make their presence more visible in a landscape dominated by faith.

Jones got the first inkling for the idea while leaving a Christmas carol concert six years ago.

"There was so much about it that I loved, but it's a shame because at the heart of it, it's something I don't believe in," Jones said. "If you think about church, there's very little that's bad. It's singing awesome songs, hearing interesting talks, thinking about improving yourself and helping other people — and doing that in a community with wonderful relationships. What part of that is not to like?"

The movement dovetails with new studies showing an increasing number of Americans are drifting from any religious affiliation.

The Pew Forum on Religion & Public Life released a study last year that found 20 percent of Americans say they have no religious affiliation, an increase from 15 percent in the last five years. Pew researchers stressed, however, that the category also encompassed majorities of people who said they believed in God but had no ties with organized religion and people who consider themselves "spiritual" but not "religious."

Sunday Assembly — whose motto is Live Better, Help Often, Wonder More — taps into that universe of people who left their faith but now miss the community church provided, said Phil Zuckerman, a professor of secular studies at Pitzer College in Claremont.

It also plays into a feeling among some atheists that they should make themselves more visible. For example, last December, an atheist in Santa Monica created an uproar — and triggered a lawsuit — when he set up a godless display amid Christian nativity scenes that were part of a beloved, decades-old tradition.

"In the U.S., there's a little bit of a feeling that if you're not religious, you're not patriotic. I think a lot of secular people say, 'Hey, wait a minute. We are charitable, we are good people, we're good parents and we are just as good citizens as you and we're going to start a church to prove it," said Zuckerman. "It's still a minority, but there's enough of them now."

That impulse, however, has raised the ire of those who have spent years pushing back against the idea that atheism itself is a religion.

"The idea that you're building an entire organization based on what you don't believe, to me, sounds like an offense against sensibility," said Michael Luciano, a self-described atheist who was raised Roman Catholic but left when he became disillusioned.

"There's something not OK with appropriating all of this religious language, imagery and ritual for atheism."

That sentiment didn't seem to detract from the excitement Sunday at the inaugural meeting in Los Angeles.

Hundreds of atheists and atheist-curious packed into a Hollywood auditorium for a boisterous service filled with live music, moments of reflection and an "inspirational talk, " and some stand-up comedy by Jones, the movement's co-founder.

During the service, attendees stomped their feet, clapped their hands and cheered as Jones and Evans led the group through rousing renditions of "Lean on Me," ''Here Comes the Sun" and other hits that took the place of gospel songs. Congregants dissolved into laughter at a get-to-know-you game that involved clapping and slapping the hands of the person next to them and applauded as members of the audience spoke about community service projects they had started in LA.

At the end, volunteers passed cardboard boxes for donations as attendees mingled over coffee and pastries and children played on the floor.

For atheist Elijah Senn, the morning was perfect.

"I think the image that we have put forward in a lot of ways has been a scary, mean, we want to tear down the walls, we want to do destructive things kind of image is what a lot of people have of us," he said. "I'm really excited to be able to come together and show that it's not about destruction. It's about making things and making things better."

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On the Web:

Sunday Assembly: http://www.SundayAssembly.com

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Follow Gillian Flaccus on Twitter at http://www.twitter.com/gflaccus