Chris Pratt and Reese Witherspoon remember victims of 9/11

Chris Pratt, Reese Witherspoon, and Cher are among the stars taking a moment to remember the lives lost in the 9/11 terrorist attacks.

Friday marked the 19th anniversary of the tragedy, when members of Islamic extremist group Al-Qaeda hijacked four U.S. passenger planes, crashing two into the World Trade Center towers in New York City, and one into the Pentagon in Virginia.

The fourth aircraft was directed to Washington, D.C., but eventually crashed in a field in Pennsylvania after heroic passengers fought back.

The terrorist attacks killed 2,977 people, and left more than 25,000 others injured, and the victims were on many stars' minds as they took to social media on Friday.

In a post on Instagram, Pratt wrote, "May we honor the fallen with our continued defense of liberty, one nation, under God, indivisible. May we continually pursue a more perfect union. And may we never forget, not only the fallen, but the values for which we stand, lest their death be in vain. #911 #september11 #unitedwestand".

"Today we honor and remember the fallen victims and heroes of 9/11. My thoughts are with all the families and friends who experienced unimaginable loss. #neverforget," added Witherspoon.

Her Little Fires Everywhere co-star Kerry Washington shared, "I'm from New York. I was built by New York. I love New York. 9/11 is always a reminder to me why I love this city and what #NewYorkStrong actually means. This year more than ever this city has had to overcome so much & the people never fail to make me proud. #NeverForget".

Jennifer Lopez, Eva Longoria, Nicole Richie, Kate Hudson, and Kris Jenner also shared tributes online, and superstar Cher tweeted: "Moment Of Silence For The People Who Died On 911 Still Brought Back Tears.The Pain Like It Was Yesterday.There Are NO WORDS (sic)".