"God is our refuge and strength, a very present help in trouble," Obama said, quoting Psalm 46. "Therefore, we will not fear, even though the Earth be removed and though the mountains be carried into the midst of the sea, though its waters roar and be troubled, though the mountains shake with its swelling."
At the somber ceremony marking the 10th anniversary of the Sept. 11, 2001, terrorist attacks, former President George W. Bush joined Obama, marking the first time the two came together at Ground Zero.
Bush read from a letter Abraham Lincoln had written to a mother who lost five sons in the Civil War and said Lincoln understood the "cost of sacrifice."
Reading from the letter, Bush said, "I pray that our Heavenly Father may assuage the anguish of your bereavement, and leave you only the cherished memory of the loved and lost, and the solemn pride that must be yours to have laid so costly a sacrifice upon the altar of freedom."
On a clear day that recalled the morning of the terrorist attacks a decade ago, thousands of people observed moments of silence at 8:46 a.m., when the first plane hit the North Tower and at 9:03 a.m., when the second plane hit the South Tower.
Relatives of victims got their first closeup look at the National September 11th Memorial site, with pools in the footprints of each towers, a 30-foot waterfall cascading down all four sides of each and names of those killed at the World Trade Center in 2001 and in a 1993 bombing inscribed in bronze panels.
As Obama walked alongside the wall of the North Tower pool, he touched names on the wall.
In the crowd near the memorial, Chundera Epps, whose brother died on the 98th floor of the North Tower, said, "When it comes to family gatherings, that's when the hurt comes in. The first Thanksgiving, all we did was cry, we couldn't even eat."
Family members began reading the names of the nearly 3,000 victims killed in New York and when planes crashed into the Pentagon and near Shanksville, Pa.
Bagpipes played, bells tolled at nearby churches, mourners carried flags or photos of those who died.
"They were our neighbors, our friends, our husbands, wives, brothers, sisters, children and parents," New York Mayor Michael Bloomberg said at the ceremony. "They were the ones who rushed in to help, 2,983 innocent men, women and children. We have asked their families to come here to speak the names out loud to remind each of us of a person we lost in New York, in Washington and Pennsylvania."
Ninety miles north of Ground Zero, below the Hudson River corridor that hijacked American Airlines Flight 11 and United Airlines Flight 175 took as they flew south to the World Trade Center, more than 100 people met at the midpoint of a wooden bridge spanning a Hudson River lagoon in Kingston, N.Y., for an "Interfaith Call to Remembrance and Healing."
"Life is but a narrow bridge. The most important thing is not to be afraid," Rabbi Yael Romer of Kingston's Temple Emanuel said to the group, quoting 19th century Ukrainian Rabbi Nachman of Breslov.
The attendees had come together at the bridge's midpoint from opposite sides as wordless Jewish and Muslim melodies were sung.
The meditative and musical event, which took place at the same time as the New York City observance, featured Western, Eastern and American Indian prayers and chants, universal spiritual intentions and communal moments of silence.
The gathering ended with the gospel children's song "This Little Light of Mine."