A somber President Barack Obama bore silent witness here Sunday as gathering clouds and raw emotion suffused the ritual of remembering loved ones lost to an unspeakable act of terror.

At the city's 10th commemoration ceremony of the Sept. 11, 2001, al-Qaida attacks, held this year at the new September 11 Memorial and Museum, the president broke his public silence just once, for two minutes out of his 90 minutes in New York.

"God is our refuge and strength, a very present help in trouble," Obama said as he read Psalm 46 at a glass-enclosed podium near the memorial. "Therefore, we will not fear, even though the earth be removed, and though the mountains be carried into the midst of the sea."

For the first time, Obama joined his predecessor, former President George W. Bush, at the 16-acre World Trade Center site, where Bush had used a bullhorn to rally first responders and other workers on the pile 10 years ago.

Obama and Bush entered the site, each holding his wife's hand, as they walked by the North Memorial Pool, with its rushing waters and victims' names etched into its sides. There, they met with a small group of family members and local officials. Former first lady Laura Bush was particularly affected, wiping her nose and eyes with tissues throughout her visit to the site, according to Politico.com.

After the first moment of silence, at 8:46 a.m., the time when Flight 11 struck the north tower, Obama stepped to the podium to read Psalm 46, which an aide said he chose for its message of persevering through challenges and emerging stronger.

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After the second moment of silence, at 9:03 a.m., when Flight 175 struck the south tower, Bush stepped to the podium and read a letter from President Abraham Lincoln to the parents of slain soldiers. "I pray that our heavenly father may assuage the anguish of your bereavement," Bush read.

For the Obamas, Manhattan was the first of three stops to mark each of the sites -- the World Trade Center, the ground at Shanksville, Pa., and the Pentagon -- where 10 years ago 19 suicidal terrorists commandeered and crashed four commercial airliners, killing 2,977 people.

Obama also was to speak at a commemoration at the Kennedy Center in Washington Sunday night.

The first anniversary of 9/11 in 2002, when Bush also visited all three sites, established the tradition of the president making no speeches until the end of the day. Bush gave an evening address from Ellis Island in 2002 and from the Oval Office for the fifth anniversary in 2006.

Obama came to Manhattan under the shadow of political woes and an unconfirmed threat of a car bomb ordered by the remnants of al-Qaida, after its leader Osama bin Laden had been killed this past spring in a raid by U.S. Navy SEALs that the president had ordered.

Obama had visited the site twice before, as a candidate with his rival Sen. John McCain (R-Ariz.) in 2008 and in May, to mark bin Laden's death.

But Sunday, the lower Manhattan that Obama encountered resembled an armed encampment, with streets and sidewalks cleared behind blue barriers, patrol cars with flashing lights and police officers posted in pairs at every corner.