"He's now slipped the bonds of Earth once again, but what a legacy he left," former Treasury Secretary John Snow said at the public interfaith memorial.
Armstrong, who died Aug. 25 at age 82 from complications resulting from cardiovascular procedures, walked on the moon in July 1969. A private service was held for Armstrong in Ohio.
"You have now shown once again the pathway to the stars," Eugene Cernan, the last man to walk on the moon said in a tribute. "As you soar through the heavens beyond even where eagles dare to go, you can now finally put out your hand and touch the face of God."
Cernan speech was followed by a slow and solemn version of the song "Fly Me to the Moon" by singer Diana Krall.
Apollo 11 crewmates Buzz Aldrin and Michael Collins and Mercury astronaut John Glenn and about two dozen members of Congress were among about 1,500 people crowded in the cathedral. A moon rock that the Apollo 11 astronauts gave the church in 1974 is embedded in one of its stained glass windows.
The service also included excerpts from a speech by John F. Kennedy 50 years ago in which he said America chose to send men to the moon not because it was easy, but because it was hard.
Armstrong commanded the historic landing of the Apollo 11 spacecraft on the moon July 20, 1969. His first words after stepping onto the moon are etched in history books: "That's one small step for man, one giant leap for mankind."
The Associated Press contributed to this report.