To chants of "USA!" and "Yes we can!" Obama and First Lady Michele Obama greeted those who visited the site but did not speak publicly.
At a ceremony earlier Sunday, Pennsylvania Gov. Tom Corbett praised the 40 passengers and crew who stopped the terrorists from flying the plane toward Washington, the Pittsburgh Post-Gazette reported.
"Nothing we build in stone and mortar can sufficiently honor the deeds," Corbett said. "I get a feeling when I go to some of those places, to Gettysburg, to the Alamo and here, a feeling of spirit, of a presence. ... But the truth is this location is like no other. There is nothing with which to compare the passenger uprising of 10 years ago."
For a second straight day, thousands gathered in the field where Flight 93 crashed after passengers and crew took a vote, stormed the cockpit and tried to take control of the plane from terrorists. They prevented from hitting its likely target, the U.S. Capitol, 20 minutes away by air.
The crowd observed a moment of silence at 10:03 a.m., when the flight crashed a decade ago.
Relatives or friends read victims' names and a bell tolled for each.
The $62 million memorial, which features a marble wall of names, is still incomplete.
"Your presence today means almost as much to the families as the memorial itself," said former Pennsylvania Gov. Tom Ridge, who was in office during the terrorist attacks and was later appointed by President George W. Bush to be the nation's first secretary of homeland security.
"You're the VIPs. Your very presence is a powerful message of comfort and love to these families and, on their behalf, I thank you."
Mary Blair, a United flight attendant who knew an attendant aboard Flight 93, said she and others think about those aboard Flight 93.
"It's a nice feeling to be here and know that they are safe and happy where they are now," she said.