The United States is marking the 15th anniversary of the September 11th terrorist attacks, when al-Qaida terrorists hijacked four planes and flew them into the World Trade Center in New York and the Pentagon near Washington, while one crashed in rural Pennsylvania.
President Barack Obama observed a moment of silence at the White House Sunday at 8:46 a.m., when the first of the four hijacked planes slammed into the World Trade Center, before speaking at a ceremony at the Pentagon, the U.S. military headquarters, honoring those killed in the attacks.
"We remember and we will never forget the nearly 3,000 beautiful lives taken from us so cruelly," Obama said. "We wonder how their lives might have unfolded, how their dreams might have taken shape."
WATCH: President Obama Apeaks at Pentagon Memorial
He vowed that terrorists "will never be able to defeat a nation as great and as strong as America," praising the country's diverse ethnic population comprised of people of all races and religions as "one of our greatest strengths."
Obama, commemorating the September 11, 2001 attacks for the last time as president before leaving office in January, said, "This is the America that was attacked that September morning. This is the America that we must remain true to."
IN PICTURES: 15th Anniversary of 9/11 Attacks
Victims killed in the attack on the World Trade Center were remembered in New York, where the country's leading 2016 presidential candidates, Democrat Hillary Clinton and Republican Donald Trump, paid their respects at Ground Zero but halted their political campaigns for the day.
As they have been in past 9/11 commemorations in New York, the names of the 2,983 killed that horrifying day were read slowly by their relatives as music played in the background.
WATCH: 9/11 Commemorations in New York
As daylight ends Sunday in New York, spotlights will project two giant beams of light into the sky to represent the fallen twin towers of the World Trade Center.
"Fifteen years ago, a September day that began like any other became one of the darkest in our nation's history," Obama said Saturday in his weekly address.
The president said those killed were "from all walks of life, all races and religions, all colors and creeds, from across America and around the world." It was the worst attack on U.S. soil since Pearl Harbor was bombed in 1941 at the start of World War II.
View the photo gallery of 9/11 attacks:
Nineteen hijackers, 15 of them from Saudi Arabia, were killed in the attacks, which led directly to the U.S. war in Afghanistan, where al-Qaida trained attackers against the United States, and indirectly to the war in Iraq. The U.S. still has thousands of forces in Afghanistan and Iraq even as it has ended large-scale combat operations.
Writing on Twitter Sunday, Secretary of State John Kerry said, "On 9/11, we remember those we lost, those who tried to save them. We honor them by pursuing peace, security, justice worldwide."
Near Shanksville in western Pennsylvania Sunday, the Flight 93 National Memorial stands in memory of the passengers and crew members who carried out a sustained assault against the hijackers for control of the plane 15 years ago.
A September 11 Museum has been erected on the New York site where the World Trade Center once stood, housing artifacts and photographs connected to the attack.
At the Pentagon, the 184 people who died on September 11, 2001 are honored with 184 benches over pools of water. A huge American flag was draped from the roof of the headquarters of the country's Defense Department on the side of the building where the attack occurred.