Farrow wrote, "On another clear blue morning, thoughts with the families & friends of those killed on 9/11."
Moore sent inspirational words to her followers. "Thoughts, prayers, love and light to those who lost ones 11 years ago today. Never forget.”
Oscar winner Octavia Spencer tweeted, "I say vics [victims] of 9/11 are heroes b/c [because] they were ordinary ppl [people] just trying to live. Their sacrifice will always be sadly heroic. Never Again!!!"
“Twilight” star Ashley Greene wrote, “Take a moment to #Remember911 and all of the loved ones lost. We are so blessed to be living in the USA.”
Joan Rivers wrote, “Today, we pay tribute to the innocent Americans’ lives lost 11 years ago. My heart goes out to the families and friends of all the victims.”
Kelly Osbourne's tweet also carried a more personal tone. "I personally went down there 2 (to) help clean up! It was a lot of work & our heroes deserve the respect! My thoughts & prays are with all the family's (sic) that lost loved ones on 911! It might have been 11 years but we never forget our loved ones!"
President Barack Obama observed one-minute of silence on the White House Lawn with wife Michelle by his side as victims' families gathered at Ground Zero in New York, the Pentagon in Washington, D.C., and near Shanksville, Pennsylvania to honor the dead.
Actress Mia Farrow and singer Mandy Moore are among celebrities who took to their Twitter accounts to pay tribute to the victims of the 9/11 terrorist attacks on the 11th anniversary of the tragedy.