At a ceremony in a state park near Aurora, where the victims were killed while watching a Batman movie, volunteers read out the names of thousands of people who had died from gun violence.
The 10-hour event ended at 12:28 a.m., the moment the shooting began a year ago, CNN reported.
"From a movie theater to an elementary school to a church, it's all different," said Carlee Soto, whose sister died five months after being wounded in the Aurora shooting. "But we all carry the same grief, and we all share the wanting to change our gun laws."
The Aurora shooting left 70 people injured.
Gun-rights advocate Rob Blanken said the shooting was a tragedy but "it's also a tragedy that a firearm was not allowed to be used inside that theater that may have prevented that tragedy."
A crowd gathered in the early Saturday morning moonlight at another memorial across the street from the Aurora theater. Teary-eyed visitors, some wearing Batman costumes, placed flowers, notes and personal items around 12 wooden crosses that represented each victim.
Just before midnight, the crowd formed a large circle, each person holding a candle. They prayed and had a moment of silence.
A long line of Aurora police cruisers drove past the memorial, emergency lights flashing, in a sign of respect.