NEW YORK — With simple and solemn ceremony, the United States marked the 10th anniversary of the Sept. 11, 2001, terrorist attacks Sunday with prayer and remembrances at the sites where thousands of Americans died on this day a decade ago.

Americans of every stripe, from presidents to firefighters to average citizens, paused to honor the dead in churches, at the sites of the attacks, and in living rooms across the country. Church bells rang. Prayers were read aloud. Choirs sang.

“God is our refuge and strength, a very present help in trouble,” President Barack Obama said at the World Trade Center site in New York, reading from 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.”

Obama was joined by former President George W. Bush in a moment of bipartisan unity reminiscent of the way the country came together in the wake of the attacks.

Bush read of sacrifice from a letter written by Abraham Lincoln to a mother who lost five sons in the Civil War.

Hand in hand with their wives, the two presidents walked to the site as the ceremony opened at 8:46 a.m., the precise moment the first hijacked plane smashed into the first tower. A church bell rang twice.

Obama’s hand grazed the new bronze panels bearing the names of the dead, part of the moving memorial formally dedicated Sunday. They went on to the North Memorial Pool, built on the footprint of the World Trade Center’s North tower.

The ceremony took place under the threat of terror attack, the audience there by invitation only, the leaders behind bulletproof glass.

Chundera Epps, whose brother was on the 98th floor of the North tower, said it still hurts 10 years later.

“When it comes to family gatherings, that’s when the hurt comes in,” Epps said. “The first Thanksgiving all we did was cry, we couldn’t even eat.”

Some wore shirts bearing images of those who perished in the attacks, or carried signs with pictures of loved ones and the words “never forget.” Surrounding the site, U.S. flags flew under a sunny sky at half mast.

Obama left the site precisely at 9:11 a.m., bound for Shanskville, Pa., to honor the passengers of United Flight 93 who died stopping their hijacked plane from reaching its intended target, the U.S. Capitol or the White House.

There, he and First Lady Michelle Obama laid a wreath at the undulating wall of white marble that is engraved with the names of those aboard Flight 93.

The Obamas also visited the boulder that marks the actual crash site, standing quietly in a field of wildflowers, gazing into the distance. They spent nearly an hour greeting guests, most of them families of passengers on the ill-fated flight.

“God bless you and our country,” Linda White of Hamburg, N.Y., said to Obama.

White told the president that her cousin, Louis J. Nacke, was among the passengers and that her husband has been at the site every year but one. That exception came the year he joined a motorcycle ride cross-country that symbolically finished the flight’s intended journey from Newark to San Francisco.

Kevin Marisay brought an American flag he carried in memory of his sister, Georgine Rose Corrigan, who had boarded Flight 93 instead of the later flight she had booked in hopes of getting home early from a business trip.

“It’s still not setting with me that she’s gone,” Marisay said.

At the Pentagon, the remembrance began at 6:48 a.m. when Pentagon workers unfurled a flag to the right of very spot where the building was struck, just as they had done on Sept. 12, 2001.

Guests observed a moment of silence at 9:37 a.m., the same minute that a hijacked plane had hit there on Sept. 11, 2001.

Vice President Joe Biden attended the Pentagon ceremony, where he praised the courage of the family members of the 184 people who died there.

“I know these memorials — and you’ve been through many — are bittersweet moments for you, because as you sit here right now, unlike a month ago, everything’s come back in stark relief. It’s not a thought, it’s precise. You remember that God-awful empty feeling, you remember being sucked into your own chest and that feeling of hollowness,” he said.

“But I want you to know something else,” he added. “Your physical presence here today gives hope to thousands of Americans who under different circumstances are trying to come to grips with the losses that you had.”