It has been exactly one week since 20 young children and 6 adults were shot and killed at Sandy Hook Elementary in Newtown, CT -- just the latest in a string of devastating, high profile tragedies caused by gun violence.

Like millions of Americans across the country, Hollywood's biggest, most talked-about stars have seen enough. On Friday Dec. 21, Mayors Against Illegal Guns released Demand a Plan to End Gun Violence, an online campaign with a powerful, moving video message -- starring a staggering roster of A-list celebrities, including: Beyonce, Jennifer Aniston, Kate Hudson, Reese Witherspoon, Jennifer Garner, Jessica Alba, Gwyneth Paltrow, Cameron Diaz, Michelle Williams, Courteney Cox, Ellen DeGeneres, Julianne Moore, Selena Gomez, Chris Rock, Jon Hamm, Will Ferrell, Aziz Ansari,  Christina Applegate, Debra Messing, Elizabeth Banks, Brooke Shields, Kristen Bell, Jamie Foxx, Sarah Silverman, Amy Poehler, Jason Bateman and others.

Recalling the slew of gun-related tragedies -- Columbine, Oak Creek, Virginia Tech and Newtown -- the stars ask in the video, "How many more . . . colleges, classrooms, houses of faith, street corners."

"Enough," the video continues. "Demand a plan. Right now. We can do better than this."

Watch the emotional, star-packed video now -- and visit now to find out more about how to sign up, spread the word to members of Congress, the Senate and President Obama.