NPR quoted organizer David Silverman, president of American Atheists and the rally organizer, as saying it marked a coming-of-age for nonbelievers.
"We'll look back at the Reason Rally as one of the game-changing events when people started to look at atheism and look at atheists in a different light," Silverman said.
The rally was expected to draw 30,000 nonbelievers, secular humanists and agnostics, according to the Washington Times.
However, with thunderstorms forecast for Saturday, Silverman told CNN that he expected a crowd of between 5,000 and 10,000 people.
Celebrity activist for atheism Richard Dawkins, scheduled to give an address at the rally, said it was tailored to a young generation that he sees as a rising secular force.
Comedian Eddie Izzard and Adam Savage, co-host of Discovery Channel’s "Mythbusters," were also on the speakers' list.
Silverman said the message of the rally was to encourage closeted atheists to take heart.
"The message is that if you can come out, you can out come out," he says. "And if you can't come out, at least you'll know you're not alone, and maybe sometime soon you'll be able to come out of the closet to your family."
CNN reported cost of the event as around $300,000, with philanthropist Todd Stiefel, Founder of the Stiefel Freethought Foundation, supplying half the money.
Meanwhile, Westboro Baptist Church — an anti-gay extremist religious group known to picket of funerals of US servicemen and women — had been granted a permit for a rally between 14th and 15th streets, CNN cited Carol Johnson of the National Park Service as saying.
CNN.com quoted Silverman as saying the rally was aimed at letting the religious know that there were nonbelievers among them, and that: "Atheism is growing in all 50 states. What people don’t seem to understand is all we demand at American Atheists is equality."
Organizer and journalist Jamilia Bey echoed his sentiments, saying the message of the gathering was: "We vote. We live here. We buy things. We matter."