The Reason Rally, sponsored by 20 atheist, secular and humanist groups, was billed as the biggest-ever "coming-out" party.
An estimated 10,000 people rallied in Washington D.C. to celebrate the rejection of the idea of God.
The fastest growing 'religious' group is campaigning for a bigger place in public life.
Deep-seated fear of discrimination leads many Americans with no religious affiliation not to acknowledge themselves as atheists.
Thousands of atheists, agnostics and other non-believers turned out in the US capital on Saturday to celebrate their rejection of the idea of God and to claim a bigger place in public life.
The Reason Rally, sponsored by 20 atheist, secular and humanist groups, was billed as the biggest-ever "coming-out" party for the fastest-growing religious group in the United States -- those with no religion.
"There are too many people in this country who have been cowed into fear of coming out as atheists, secularists or agnostics," said the event's star turn, Richard Dawkins, the British scientist and best-selling atheist author.
"We are far more numerous than anybody realizes," he said, prompting a loud cheer from the youthful crowd that defied gray skies and drizzle for an afternoon of speeches, music and satire on the National Mall.
Jesse Galef of the Secular Student Alliance, a spokesman for the rally, told AFP he conservatively estimated the turnout at 10,000. The National Park Service, which oversees the mall, had issued a permit for 15,000.
In the center of the good-humored crowd rose a crucifix with an affixed sign that declared: "Banish the Ten Commandments to the dustbin of history." Other posters read: "Good without a god" and "Hi Mom! I'm an atheist."
"This country was not built on religion and God," said another of the day's speaker, Michael Shermer, a self-defined "skeptic" and columnist for the respected Scientific American magazine. "It was built on reason."
"God fixation won't fix our nation, because nothing fails like prayer," added Annie Laurie Gaylor of the Freedom From Religion Foundation, which is contesting Pennsylvania's declaration of 2012 as "the year of the Bible."
On the edges of the Mall, atheists engaged in vigorous debates with a handful of Christians who turned up with their own placards that read: "Study and obey the Bible" and "Jesus forgives sin."
"Jesus Christ is your only hope," exclaimed one soapbox preacher through a bullhorn. "Humble yourself today."
In no other Western country does religion figure so highly in society as in the United States, where "In God We Trust" appears on bank notes and "one nation under God" is part of the national Pledge of Allegiance.
Yet the most recent American Religious Identification Survey, published in 2009, found that Americans with no religious affiliations -- "the nones" in sociological jargon -- make up 15 percent of the total adult population.
"That is more than Jews, Muslims, Hindus and Buddhists combined and doubled," said David Silverman, president of American Atheists, which campaigns for the civil rights of non-believers.
David Roozen of the Hartford Institute for Religion Research in Connecticut said the number of Americans with no religious affiliation has "about doubled" in the last 20 years.
"It's probably the fastest-growing category of religion in the United States," the sociologist told AFP in a telephone interview earlier this week.
Silverman, who defines atheism simply as "the lack of belief in a god," said a deep-seated fear of prejudice and discrimination leads many Americans with no religious affiliation not to acknowledge themselves as atheists.
Such discrimination, atheists say, includes a virtual inability to serve in public office, the risk of being fired by a religiously devout employer, denial of reproductive health care and religiously biased school texts.
"These are battles that homosexuals have won, people of color have won, women have won," said journalist Jamila Bey, who recalled losing a job after her Christian boss learned she was an atheist. "We can't stay silent anymore."