Attendees sang and swayed to Christian music blared out on the National Mall, along with addresses by Christian pastors.
Defence secretary Pete Hegseth andspeaker of the House Mike Johnson are listed as guest speakers. President Donald Trump and secretary of state Marco Rubio will address the crowd via video.
The gathering was organised by the White House as part of a programme of celebrations for America’s 250th anniversary and, in a video message inviting Americans to attend, Hegseth said it was an opportunity to “rededicate this republic to God and country.”
Muscular Christian nationalism has enjoyed a prominent platform since Trump’s return to power, and evangelicals form a core element of the president’s support base.
Hegseth is a member of an ultra-conservative evangelical church, and his briefings on the Iran war have been notable for their use of bellicose, Christian rhetoric.
“Today, friends, we are in a spiritual war. This is a battle in our day between good and evil, between right and wrong, between truth and lies, between light and darkness,” Pastor Gary Hamrick of Virginia told the crowd.
“This is a battle for the very soul of America.”
The US Constitution explicitly bars the establishment of any official religion, but the expression of any faith is also explicitly protected.
“We came here to rededicate our country back to God. Our country has fallen away in so many areas,” Jeana Dobbins, a 67-year-old retiree who travelled from North Carolina with her friend, told AFP.
Attendee Sarah Tyson, holding a “Jesus Saves” sign, said she believes Trump was chosen by God to lead the nation through a new spiritual revival.
“God ordained him for a time like this, because these United States need to wake up,” said Tyson, a middle-aged woman who came from New York with fellow church members.
While previous administrations and presidents have regularly held and attended faith-based gatherings, Sunday’s event is still unusual for its scale and the presence of top cabinet officials.
And apart from a rabbi and a retired Catholic archbishop, almost all the 20 listed “faith leaders” who will speak are evangelical Protestants.
“It’s not unprecedented to have a group of evangelical pastors or conservative clergy come together for something like this and blend a certain kind of nationalism with a certain kind of conservative Christianity,” said Sam Perry, a professor at Baylor University, a Christian school in Texas.
But “the Trump administration taking the lead on this celebration at this scale is different than previous events,” Perry added.
The organisers’ website says the prayer gathering is for “Americans of every background” but Julie Ingersoll, a professor of religious studies at the University of North Florida, says the list of speakers suggests “an idea of American identity that is rooted in whiteness and Christianity.”
The event “sends a specific message… that they are the mainstream Americans, and the rest of us are sidelined,” Ingersoll said.
The National Mall, which stretches from the US Capitol to the Lincoln Memorial, is a common site for mass rallies and protests – most famously the 1963 March on Washington, when an estimated 250,000 people heard Martin Luther King Jr deliver his “I Have a Dream” speech.
Sunday’s gathering is scheduled to last around nine hours.