The massacre took place at a showing of The Dark Knight Rises, in the town of Aurora, near Denver, where a masked man wearing body armour entered the cinema through an emergency exit and set off tear gas or smoke bombs before opening fire in the packed auditorium.
Early reports suggested 14 people had died, but officials have since confirmed the death toll is 12. Up to 50 others, including four children, have been injured.
The atrocity has stunned America, and U.S. leader President Obama released a statement which read, ""Michelle and I are shocked and saddened by the horrific and tragic shooting in Colorado... We are committed to bringing whoever was responsible to justice, ensuring the safety of our people, and caring for those who have been wounded. As we do when confronted by moments of darkness and challenge, we must now come together as one American family.""
At a rally in Fort Myers, Florida later on Friday, Obama held a moment's silence for the victims and told the crowd, ""We're gonna stand by our neighbours in Colorado during this extraordinarily difficult time. Such violence, such evil is senseless, it's beyond reason, but while we will never know fully what causes somebody to take the life of another, we do know what makes life worth living.
""The people we lost in Aurora loved and they were loved. They were mothers and fathers, they were husbands and wives, sisters and brothers, sons and daughters, friends and neighbours, they had hopes for the future and they had dreams that were not yet fulfilled.""
A string of shocked stars took to Twitter.com on Friday to express their outrage at the shootings, and to pass on their condolences to those involved.
Eva Longoria writes, ""My heart goes out to all the people affected in the Aurora, CO shooting. May God be with that town and their families.""
Whoopi Goldberg adds, ""A mad man has shattered lives forever in Colorado. To everyone who helped folks get to safety in that theatre THANK YOU for your bravery.""
Paramore singer Hayley Williams, who was watching the film at a different cinema, tweeted, ""In shock reading about what happened at the theater in Aurora, CO... Could have been any of us. My heart is heavy for everyone affected.""
Film director Kevin Smith writes, ""My heart goes out to all the families of the victims in the Aurora, Colorado movie theater shooting. F**king horrific.""
Billy Ray Cyrus also took a moment to reflect on the tragic news, adding, ""Oh no! Just woke up and turned on the news... Praying for victims. So sorry and sad.""
In the wake of the incident, bosses at Warner Bros., the studio behind the film, have cancelled the movie's French premiere in Paris on Friday night (20Jul12).
A spokesperson for the company says, ""Warner Bros. is deeply saddened to learn about this shocking incident. We extend our sincere sympathies to the families and loved ones of the victims at this tragic time.""
Suspect James Holmes, 24, has been taken into custody over the shootings.