"No turning of the seasons can diminish the pain and the loss of that day," Obama said at the Pentagon Memorial. "No passage of time and no dark skies can ever dull the meaning of this moment. We remember with reverence the lives we lost. We read their names. We press their photos to our hearts. And on this day that marks their death, we recall the beauty and meaning of their lives."

Unprotected from the rain, he invoked Scripture, saying, "The mountains may fall and the earth may give way; the flesh and the heart may fail. But after all our suffering, God and grace will 'restore you and make you strong, firm and steadfast.' So it is -- so it has been for these families. So it must be for our nation.. on a day when others sought to sap our confidence, let us renew our common purpose. Let us remember how we came together as one nation, as one people, as Americans, united not only in our grief, but in our resolve to stand with one another, to stand up for the country we all love."

"Through their own lives," he added, "the men and women who lost their lives eight years ago today leave a legacy that still shines brightly in the darkness, and that calls on all of us to be strong and firm and united. That is our calling today and in all the Septembers still to come."

Pentagon was where Flight 77 crashed, killing all 59 people on board and 125 military and civilian personnel. The memorial near its west wall consists of 184 stainless steel benches, each inscribed with a name of a victim and constructed over a small pool of water.

Earlier that day, the President stood in silence on the South Lawn of the White House with First Lady Michelle Obama and 200 staff to mark the moment when the first plane hit the World Trade Center. A bell rang three times before the minute of silence, after which a bugler played taps while all placed their hands on their hearts.

Obama also penned an op-ed in the New York Daily News that said, "Every year on this day, we are all New Yorkers... On this day, and every day, it is incumbent on each of us to uphold those ideals that our enemies were - and are - so eager to destroy. To serve others and give back to our communities. To respect our differences and to value what we share. To remember that even when we disagree, and disagree strongly, that we are all Americans."

In New York, Biden and Second Lady Jill Biden will take part in the city's September 11th Commemoration Ceremony at Zuccotti Park. Names of the victims at the World Trade Center were read following tradition, but this year the reading was done by family members paired with volunteers to commemorate the day as a National Day of Service and Remembrance.

There was also a ceremony in Pennsylvania, where 40 people died after Flight 93 was thwarted by passengers from hitting the U.S. Capitol and instead crashed on Skyline Road in Shanksville. Former Secretary of State Colin Powell and former Homeland Security Sec. Tom Ridge spoke at the event.

Four planes hijacked by terrorists crashed into the twin towers of the World Trade Center, the Pentagon and in Pennsylvania on Sept. 11, 2001. A total of 2,973 people died. The United States launched its War on Terror and invaded Afghanistan the next month in pursuit of al Qaeda and Osama bin Laden.