Amidst thousands of onlookers including President Obama, the family members of 9/11 victims have presented the names of those lost.
Americans are commemorating the heroism of many that day 10 years ago, and the selfless acts of the first responders that arrived on the scenes of the terrorist attacks in New York, Washington, and Pennsylvania.
A moment of silence was held in New York City at 8:46 a.m.; the time when American Airlines flight 11 hit the North Tower of the World Trade Center.
Across Lower Manhattan, the sound of churches bells rang out to mark the moment.
After Obama reading Psalm 46, 167 pairs of family members began reading the names of those who died that day.
The reading was stopped for another moment of silence at 9:03 a.m. -- the time when United Airlines Flight 175 smashed into the South Tower.
Former presidents George Bush as well as George W. Bush attended, alongside former First Lady Barbara Bush, who was viewed dabbing her eyes with a tissue.
Regular Sunday morning television was replaced by coverage of the commemorations, and across Lower Manhattan thousands gathered to watch on big screens and take part in the memorial amid an immense police presence.
Family members of victims were seen finding the names of their loved ones along the brand new memorial monuments at the World Trade Center site.
Ten years ago New Yorkers initially thought that the first crash had been a terrible accident. Few saw the plane collide with the North Tower, between the 93rd and 99th floors. However, when the second plane hit, a sudden realization of what was happening dawned upon America and the world.
In Washington, a moment of silence was followed by remarks from Joint Chiefs of Staff Chairman Adm. Mike Mullen, Vice President Joe Biden and Secretary of Defense Leon Panetta.
"Those in this building that day knew what they were witnessing," Biden said. "It was a declaration of war by stateless actors bent on changing our way of life, who believed these horrible acts, these horrible acts of terror directed against innocents could buckle our knees, could bend our will, could begin to break us and break our resolve. But they did not know us. Instead, that same American instinct that send all of you into the breach between the fourth and fifth corridors galvanized a new generation of patriots, the 9/11 generation."
"No memorial, no ceremony, no words will ever fill the void left in your heart by (the victims') loss.
“My prayer for you is that 10 years later, when you think of them ... that it brings a smile to your lips before it brings a tear to your eye."