WASHINGTON, Sept. 11 (UPI) -- President Barack Obama called on Americans to "renew our common purpose" Friday as he marked the eighth anniversary of the Sept. 11, 2001, terror attacks.

Speaking under rainy skies at the Pentagon, where 184 people died when American Airlines Flight 77 slammed into the facility, Obama said, "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."

Obama, who designated Sept. 11 as a national day of remembrance and service, said coming together as a nation is "the strongest rebuke to those who attack us" as well as paying the "highest tribute" to those who died.

"That is our calling today and in all the Septembers still to come," he said.

Navy Adm. Michael Mullen, chairman of the Joint Chiefs of Staff, said Americans shared "common sorrow" for the lives lost in Washington, New York and Pennsylvania.

"Our grief is real and it is warranted," he said. "But if I may, let me ask and let me urge that we look upon this day not only with sorrow, but also with hope for the future that those we honor wanted us to have, and gratitude for the life they wanted us to live."

Words are inadequate to "remove the pain of that loss," Defense Secretary Robert Gates said, but "in the lives of those patriots, we can find some solace."

After their remarks, Obama, Mullen and Gates participated in a wreath-laying ceremony.