WATCH: President Barack Obama and ex-President George W. Bush joined the friends and families of victims of the 9/11 attacks for a ceremony at the World Trade Center in New York. (Reuters video)
The United States is marking the 10th anniversary of the terror attacks of September 11 with solemn ceremonies to remember the nearly 3,000 people who lost their lives in New York City, Washington, D.C., and a rural field in Shanksville, Pennsylvania.
At Ground Zero in New York City, President Barack Obama and former President George W. Bush stood together in silence with heads bowed as a church bell rang twice at the exact moment a passenger jet was flown into the first tower of the World Trade Center 10 years ago in the worst terrorist attack on U.S. soil.
Obama read from the Bible's Psalm 46: "God is our refuge and strength. A very present help in trouble. Therefore we will not fear, even though the earth be removed, though the mountains be carried into the midst of the sea."
Special Report: The 9/11 Decade
Bush, who was in his first year in the White House when the attacks happened, read a Civil War-era letter from President Abraham Lincoln to a mother who lost all five of her sons.
"I pray that our Heavenly Father may assuage the anguish of your bereavement and leave you only the cherished memory of the loved and lost and the solemn pride that must be yours to have laid so costly a sacrifice upon the alter of freedom," he read.
9/11 Memorial Unveiled
Family members of those who were killed got their first look at the newly completed Memorial Plaza, which consists of two massive square pools of water meant to evoke the shape of the Twin Towers' footprints.
Water cascades from the edges in a gentle, 10-meter curtain into a smaller, center void. The names of the dead are etched in bronze plates surrounding each pool.
The 9/11 memorial is surrounded by a construction project where gleaming new office towers and a public transportation hub are being built. The centerpiece skyscraper, One World Trade Center, is partially completed and will be the tallest building in the country when it's finished.
Cellist Yo-Yo Ma played Bach's mournful suite No. 1 in G Major before the heart of the ceremony began -- a reading of the names of the dead by family members.
Photo Gallery: 9/11 attacks remembered In New York and beyond:
New York Fire Department Battalion Commander Tom Currao raises a flag outside the World Trade Center construction site at the 9/11 Memorial Visitors Center in New York.
Families line up against the wall of the South Memorial pool during 10th anniversary ceremonies at the site of the World Trade Center in New York.
U.S. President Barack Obama, First Lady Michelle Obama, and ex-President George W. Bush visit the North Memorial Pond on the 10th anniversary of the 9/11 attacks at the National September 11 Memorial at Ground Zero in New York City.
President Barack Obama, First Lady Michelle Obama, and ex-President George W. Bush visit the North Memorial Pond as they arrive for a commemoration ceremony.
New York Police Department officers check bags during the morning commute at Grand Central Terminal in New York City.
The September 11 attacks are commemorated at Manas Transit Center near Bishkek, Kyrgyzstan.
Family members who lost loved ones gather at the North pool of the 9/11 Memorial during 10th anniversary ceremonies at the site of the World Trade Center in New York.
President Barack Obama, his wife Michelle Obama, ex-President George W. Bush, and his wife Laura Bush observe a moment of silence at the time the first hijacked airliner crashed into the North Tower in 2001.
A U.S. flag recovered from the 9/11 attacks is displayed by New York City Police officers and firefighters during the ceremony marking the 10th anniversary of the terrorist attack.
Roses are stuck in the name of Richard Joseph Klares, who was killed in the attack on the World Trade Center, at the National 9/11 Memorial site in New York.
Relatives of 9/11 attack victims gather next to the memorial pool during ceremonies marking the 10th anniversary of the World Trade Center attacks in New York.
People walk along construction fences surrounding the 9/11 Memorial site in New York.
Iranians hold a candlelight vigil in Tehran for the victims of the 9/11 attacks.
Valery Savinkin lost his son, 21-year-old Vladimir Savinkin, who was an accountant for Cantor Fitzgerald and worked in the World Trade Center's North Tower, in the attacks.
"Those who were brought together by grief after September 11, 2001, are now brought together by the memory of that tragedy," he said. "But most of all by the memory of their loved ones and their desire to nurture that memory."
New York City Mayor Michael Bloomberg told the crowd of mourners that the past 10 years had been a decade of healing for the living, as well as an opportunity to honor the dead.
"Ten years have passed since a perfect blue-sky morning turned into the blackest of nights," Bloomberg said. "Since then we've lived in sunshine and in shadow and although we can never unsee what happened here, we can also see that children who lost their parents have grown into young adults, grandchildren have been born, and good works and public service have taken root to honor those we loved and lost."
In Washington, Vice President Joseph Biden, Defense Secretary Leon Panetta, and Joint Chiefs Chairman Mike Mullen joined more than 1,000 relatives of those who died on September 11 when a hijacked plane smashed into the side of the Pentagon, killing 59 passengers and 125 people inside the building.
Mullen offered his condolences to the families, who sat in rows of metal chairs facing the outdoor Pentagon 9/11 Memorial near where the plane struck.
"No music can assuage, no tongue can express, no prayer alone may dampen the yearning that must fire yet inside you," Mullen said. "Lives ended in this place. Dreams were shattered. Futures were instantly altered. Hopes were tragically dashed."
Panetta praised members of the U.S. military who have served during the country's longest time of war. "A generation of Americans stepped forward to serve in uniform, determined to confront our enemies and respond to them swiftly and justly," he said.
President Obama later laid a wreath at the memorial.
Flight 93 Remembered
After the morning ceremony in New York, Obama traveled to western Pennsylvania, where families of passengers from United Flight 93 were gathered. There, he laid a wreath at a memorial and spoke to family members.
An earlier ceremony was held on September 10 to honor the 40 passengers and crew who fought back against their hijackers aboard Flight 93 on September 11, 2001.
The plane crashed nose-first into the field after passengers successfully diverted the flight path, which is now widely thought to have been the U.S. Capitol, where both houses of Congress were in session.
At the ceremony, former President Bush said the action by the small group of Flight 93's passengers "ranks among the most courageous acts in American history."
"The moment America's democracy was under attack our citizens defied their captors by holding a vote," Bush said, referring to the hushed and quick decision they made in the back of the plane to storm the cockpit.
Former President Bill Clinton also spoke, and said the plane's passengers and crew "saved the Capitol from attack" and denied Al-Qaeda the symbolic victory of "smashing the center of American government."
Obama: 'We Remain Vigilant'
On the eve of the anniversary, the White House issued a video message from Obama, who reminded Americans that the mastermind of the 9/11 attacks, Osama bin Laden, has been killed, along with many senior Al-Qaeda leaders. He said the terror group "is on the path to defeat."
But his message came as New York and Washington were on high security alert after intelligence officials determined there was a "credible threat" of an attack on the anniversary.
"They want to terrorize us but as Americans we refuse to live in fear," Obama said. "Yes, we face a determined foe, and make no mistake, they will keep trying to hit us again. But, as we are showing again this weekend, we remain vigilant. We are doing everything in our power to protect our people and no matter what comes our way, as a resilient nation we will carry on."
Speaking at a 9/11 tribute event at the John F. Kennedy Center for the Performing Arts later, Obama ranked the battle against terrorism as among the most serious challenges that America has overcome in its history. Referring to future generations who will visit memorials to those who died on September 11, 2001, and in the subsequent wars in Iraq and Afghanistan, Obama said that "nothing can break the will of a truly United States of America."
"They will remember that we have overcome slavery and civil war, we have overcome bread lines and fascism, and recession and riots, and communism and, yes, terrorism," Obama said. "They will be reminded that we are not perfect -- but our democracy is durable."
Remembered Around The World
Americans across the country were holding their own remembrance ceremonies at churches, mosques, and firehouses.
At the site of a proposed Islamic center in lower Manhattan, people gathered to honor families of the 9/11 victims and recognize a decade of efforts to build bridges between non-Muslims and Muslims in America.
Imam Feisal Abdul Rauf, the chairman of the group behind the controversy-generating mosque and cultural center, said the event would honor 10 families of 9/11 victims -- including five Muslim victims -- to commemorate a decade of efforts to heal divisions between American Muslims and non-Muslims.
Memorials have also been held around the world. In Japan, people gathered to lay flowers before a glass case containing a small section of steel from the World Trade Center, to remember the 23 employees of Fuji Bank who lost their lives on September 11.
A village in the Philippines offered roses, balloons, and prayers for an American victim whose husband built 50 brightly colored homes there in her memory to fulfill his late wife's wish to help the Filipino poor.
But in a stark reminder that Americans are still fighting one of the wars that began in the wake of the attacks, the day also brought news that 77 U.S. soldiers were wounded and two Afghans were killed when a Taliban suicide bomber detonated a truck bomb outside the gates of a U.S. base in eastern Afghanistan.
with agency reports