"I was looking forward to having a fun conversation with you," he said. "This morning we woke up to news of a tragedy that reminds us of all the ways we are united as one American family."
"We're going to stand by our neighbors in Colorado during this extraordinarily difficult time," he told the crowd. "Such violence, such evil is senseless; it's beyond reason."
But, he added, "We do know what makes life worth living. People in Aurora loved and were loved."
Police say that a gunman killed 12 people and injured at least 50 in a suburban Denver movie theater showing "The Dark Knight Rises."
"If there's anything to take away from this tragedy, it's the reminder that life is very fragile here, time here is limited and it is precious," the president said. "What matters at the end of the day is not the small things; it is not the trivial things ... Ultimately it is how we choose to treat one another and how we love one another."
He then turned to the subject of his children. "My daughters go to the movies. What if Malia and Sasha had been in the theater as so many of our kids do every day?" He said he would hug his daughters "a little tighter" tonight.
"There are going to be other days for politics; this is a day for prayer and reflection," he said and then led a moment of silence.
Both the Obama and Mitt Romney presidential campaigns moved to halt their negative TV advertising following the tragedy. Obama will return to Washington on Friday, canceling further campaign events.
First Lady Michelle Obama and Vice President Joe Biden canceled their events on Friday in the wake of the tragedy.
Former Massachusetts Gov. Mitt Romney addressed the shootings in Bow, N.H, on Friday, calling them a "few moments of evil."
"This is a time for each of us to look into our hearts and remember how much we love one another. And how much we love, and how much we care for our great country," he said.
This post has been updated with Mitt Romney's remarks.