Lady Gaga, Miley Cyrus, More Remember September 11

Twitter is full of remembrances today 12 years after the attacks.

Twelve years ago today (September 11) Islamic terrorist group al-Qaeda hijacked four passenger airliners and flew them into the World Trade Center in New York City, the Pentagon and a field in Pennsylvania. Tuesday, Americans of all ages and professions are taking to social media to commemorate those we have lost, as well as to remember.

Those who live or have lived in New York City expressed pride in their city in the wake of the anniversary of the attacks. Ben Stiller tweeted, "Glad to be in NY on 9/11. Remembering all those who lost someone," while Joan Rivers tweeted a picture in which she is flanked by firemen, saying, "Very proud to be with two of New York's Finest on 9-11."

Others remembered what the New York skyline looked like before the attacks, tweeting images that show off the stark contrast. "This picture is actually heart breaking to look at," said Twitter user @PrettyTamar, sharing a photo from the blog Dear Photograph, while @Eaveysmom shared a John Lennon quote and another image of the city: "Imagine all the people, Living life in peace -John Lennon," she tweeted.

Still others came out to offer more general condolences; Lady Gaga tweeted, "12 years ago today Sept 11th. We Will Never Forget. Still remember the smoke in the sky. Saying lots of prayers for families who lost lives," while Miley Cyrus asked people to take action: "Do a good deed today in observance of @911day. Make your pledge at . #911day"

Most people, like Aaron Pauley of Of Mice & Men, however, just wanted to remember: "I'll never forget where I was on 9/11/01, and I'll never forget how it made me feel. Some things you carry for the rest of your life," he said. Actor Kevin Zegers also shared a memory, tweeting: "I remember sitting in calculus class 12 years ago on this day. Then running home and watching Peter Jennings for 2 straight day. #9/11"

Amber Patrice Riley also weighed in, tweeting: "I will never forget what I was doing on 9/11. Getting ready for school, the news was on but I thought I was watching 'Independence Day'..."

Many Twitter users opted to use the #wherehwereyou hashtag to share their memories, some, like @HerWarToo, writing lengthier remembrances and sharing them via image. Others, like @joeysometimes, posting personal photos.

How are you remembering September 11 today? Share with us in the comments below.