NEW YORK (Reuters) - Memorial ceremonies marking the anniversary of the September 11 attacks began Tuesday under clear blue skies that recalled the crisp morning 11 years ago when nearly 3,000 people were killed by airliners hijacked by Islamist militants.

Two of the jets brought down the Twin Towers of New York City's World Trade Center, another extensively damaged the Pentagon outside Washington and a fourth crashed in a field in rural Pennsylvania when passengers aboard that flight revolted against the hijackers.

At Ground Zero in New York where the towers once stood, more than 1,000 relatives of those killed and others gathered for the annual reading of the list of 2,983 people killed at the three sites. The list excludes the 19 hijackers, who also died.

The reading began at 8:39 a.m. (1239 GMT), with pauses for moments of silence at 8:46 a.m., 9:03 a.m., 9:37 a.m. and 10:03 a.m., the time of impact for the four planes, and again at 9:59 a.m. and 10:28 a.m., the times that the north and then the south tower fell.

As the moment of the reading approached, family members, uniformed police and firefighters milled about the vast, twin reflecting pools that mark the footprints of the two towers, their edges etched with the names of the victims. Many carried or wore pictures of their loved ones.

Alyson Low, 41, of Fayetteville, Arkansas, carried a picture of her sister, Sara Elizabeth Low, who was a flight attendant on American Airlines Flight 11, the first plane to crash, striking the trade center's north tower.

"I'm tired," Low said, tearfully. "I am just so tired."

The reading of names began with Patricia Abbott, wife of Alan Jay Richman, who died at the trade center. It will take more than 3 hours by 198 people to read the list alphabetically.

FAMILIES ONLY

In previous years, politicians including U.S. presidents, governors and New York City mayors have participated in the reading of the names, or have read from the Bible or recited passages from literature.

This year only the families of the more than 2,750 who were killed at the World Trade Center will appear on the podium to read their names.

Politicians may still attend, but under event rules set down in July by the National September 11 Memorial and Museum, chaired by New York Mayor Michael Bloomberg, none may speak or participate in the reading of names.

Homeland Security Secretary Janet Napolitano will attend the New York ceremony this year.

The restrictions will not extend to politicians at the other remembrances.

President Barack Obama observed a moment of silence for the September 11 victims on the South Lawn of the White House.

Flanked by a flag-bearing military honor guard, the president and first lady Michelle Obama stood solemnly with heads bowed.

They placed their right hands over their hearts while a bugler played "Taps." Some in the crowd of about a hundred had tears in their eyes. The Obamas then turned and, holding hands, walked back into the White House residence.

Vice President Joe Biden was in attendance to deliver remarks in Shanksville, Pennsylvania, where 40 passengers aboard United Flight 93 were killed when that plane crashed as they fought back against their hijackers.

"How we handle the legacy of these 40 people and what they did, what they kept from happening, is really more of a statement about ourselves, about what we value as a society," said Patrick White, president of Families of Flight 93.

White's cousin, Joey Nacke II, was among the passengers who stormed the cockpit.

U.S. authorities say the hijackers planned to crash that plane into the U.S. Capitol in Washington.