Al Qaeda's leadership has been devastated and Osama bin Laden will never threaten us again," President Barack Obama said Tuesday at a ceremony commemorating the 11th anniversary of the terrorist attacks of Sept. 11, 2001.

The president and first lady Michelle Obama had come to the memorial ground honoring the 146 people who died when a hijacked commercial airliner crashed into the Pentagon.

Memorial ceremonies were held Tuesday in Washington, New York and Pennsylvania to recall the worst terrorist attacks in the history of the United States, which left more than 3,000 people dead and injured.

"(T)he rest of us cannot begin to imagine the pain you've endured these many years," Obama said to the families of those who died in the attacks.

"But no matter how many years pass, no matter how many times we come together on this hallowed ground, know this - that you will never be alone," the president said.

The Sept. 11 attacks spurred Washington to invade Afghanistan, where Al Qaeda had its base of operations, in what has become the longest war in U.S. history.

Obama also paid tribute to the more than 5 million Americans who have served in uniform over the past decade.

"These men and women have done everything that we have asked," he said.

Before the ceremony at the Pentagon, the Obamas stood in front of the White House to observe a minute of silence at 8:46 a.m., the moment 11 years ago when an American Airlines plane seized by five terrorists crashed into the North Tower of the World Trade Center in New York.

At 9:03 a.m. a United Airlines jet hit the South Tower.

Another minute of silence was observed at 9:37 a.m., the time another American Airlines plane struck the Pentagon.

Vice President Joe Biden attended Tuesday in Pennsylvania the ceremony recalling the seven crew members and 33 passengers on a United Airlines plane that took off from Newark airport and crashed after some of the passengers tried to overcome the four terrorists who had taken control of the aircraft.

Republican presidential candidate Mitt Romney issued a statement to mark the sad anniversary.

"Eleven years ago, evil descended upon our country, taking thousands of lives in an unspeakable attack against innocents," he said. "America will never forget those who perished. America will never stop caring for the loved ones they left behind. And America shall remain ever vigilant against those who would do us harm." EFE