The service featured prayers led by clergy and hymns, and offered comfort and solace to the community still reeling from Friday's movie theater rampage.
"While our hearts are broken, our community is not," said Aurora Mayor Steve Hogan. "We will take this experience and use it to strengthen our commitment to each other."
Colorado Gov. John Hickenlooper also addressed the crowd of thousands and drew a strong ovation when he made it a point of saying he would not refer to shooting suspect James Holmes by name, only as "Suspect A."
The governor read aloud the names of the 12 people who were killed in Friday's rampage. After each name, the audience chanted, "We will remember."
The service concluded with a rendition of "Amazing Grace."
Earlier in the evening, President Barack Obama arrived in Aurora to meet with grieving families who lost loved ones.
"I confessed to them that words are always inadequate in these situations but that my main task was to serve as a representative of the entire country and let them know we are thinking of them at this moment," the president said at a press conference in the evening.
Mr. Obama said he also heard remarkable stories of survival from victims at the hospital.
"It reminds you that even in the darkest of days, life continues and people are strong and people bounce back and people are resilient," he said.
The president did not attend the vigil later in the evening.
The shooting by the suspected gunman, 24-year-old James Holmes, left 12 people dead and 58 others injured.
Investigators insist that Holmes acted alone in the shooting. There were reports of a second suspect, but police say they were inaccurate.
Authorities are talking to Holmes' family, friends and former classmates, hoping to learn more about his past. They're also continuing to look for clues inside his booby-trapped apartment, which was rigged with dozens of bombs.
"Make no mistake. This apartment was designed, I say, based on everything I've seen, to kill whoever entered it," described Aurora Police Chief Dan Oates at a Saturday news conference. "And who was most likely to enter that location after he planned and executed this horrific crime? It was going to be a police officer."
Read statements from public officials about the Colorado theater shooting
Investigators spent hours Saturday removing explosive materials from inside Holmes' apartment. Officials said Holmes received several mail deliveries over four months to his home and school and bought thousands rounds of ammunition on the Internet."What we're seeing here is evidence of some calculation and deliberation," Oates added.
Holmes was carrying several weapons when he was arrested after the shooting. Oates said all of the weapons and ammunition were purchased legally. A federal law enforcement official told the Associated Press that one of Holmes' weapons, a semiautomatic assault rifle, jammed during the theater attack - forcing him to switch to another gun with less firepower. This malfunction might have saved some lives.
Less than a month before the shooting, Holmes reportedly applied to join a Colorado gun range but never became a member because the owner became concerned over a "bizarre" message and behavior. The Associated Press reports that Lead Valley Range owner Glenn Rotkovich received an emailed application on June 25, and he didn't see any overt warning signs at that point.
Holmes said he was not a user of illegal drugs or a convicted felon, so Rotkovich followed up by calling Holmes' apartment to invite him to a mandatory orientation the following week.
Rotkovich got Holmes' answering machine and says "it was bizarre - guttural, freakish at best." Rotkovich left two other messages but eventually told his staff to watch for Holmes at the July 1 orientation and not to accept him into the club.
Inside Holmes' apartment, FBI Special agent James Yacone said bomb technicians neutralized what he called a "hypergolic mixture" and an improvised explosive device containing an unknown substance. There also were multiple containers of accelerants.
By late Saturday afternoon, investigators said all hazards had been removed from Holmes' apartment and residents in surrounding buildings were allowed to return home. However, those living in Holmes' apartment building were not allowed to return, because investigators were still collecting evidence. About 8 p.m. Saturday, police left the apartment building carrying a laptop computer and a hard drive.
Also, ABC News reporter Pierre Thomas told George Stephanopoulos on "This Week" that a Batman poster was removed from the apartment. According to the network, Holmes told authorities after the shooting that he was "The Joker." The Joker, played by the late Heath Ledger, was the villain in the second film of director Chris Nolan's Batman trilogy.
Police say Holmes set off gas canisters and opened fire in a packed auditorium minutes into a midnight premiere of the "The Dark Knight Rises" at Century 16 Movie Theaters at the Aurora Town Center.
See an outline of events and facts surrounding the Colorado theater shooting
Meantime, there is a growing memorial to the victims outside the theater where the shooting happened Friday. Mourners are leaving flowers, teddy bears and words of condolences. Many of the survivors are speaking out about what they went through.
Speaking from her hospital bed, injured victim Christina Blach said she will eventually forgive the shooter, but not right now.
"He killed somebody I cared about and he injured a heck of a lot of my friends. So for right now, no, down the road, yeah," Blach said.
Holmes remains in solitary confinement for his protection at a Denver-area detention facility, held without bond on suspicion of multiple counts of first-degree murder. He is set to make his first court appearance Monday morning and has been appointed a public defender. Police will not comment about Holmes' level of participation in their investigation so far - it's unknown if he's talking with officers. He did reportedly tip off officers soon after his arrest Friday, telling them about the explosive booby traps rigged in his apartment.
As the investigation continues, details about Holmes' background as a student and would-be scientist are trickling out.
Read more information about shooting suspect James Holmes
In video obtained exclusively by ABC News, Holmes is shown giving a lecture at a science camp held at Miramar College in San Diego six years ago. The then-18-year-old Holmes talks about "temporal illusions."
"Over the course of the summer I've been working with a temporal illusion. It's an illusion that allows you to change the past," Holmes said in the video.
In the footage, he appears slightly nervous speaking to the group. He is introduced at the seminar as someone whose "goals are to become a researcher and to make scientific discoveries. In personal life, he enjoys playing soccer and strategy games and his dream is to own a slurpee machine."
Holmes had recently withdrawn from a competitive graduate program in neuroscience at the University of Colorado Denver, where he was one of six students at the school to get National Institutes of Health grant money. The university said Holmes gave no reason for his withdrawal, a decision he made in June.
The university said Sunday that it was looking into whether Holmes used his position as a graduate student to order materials in the potentially deadly booby traps that police said they found in his apartment. Holmes got deliveries over four months to his home and school, authorities have said. The university is looking into what was received at the school to assist police with their investigation.
He also worked as a cabin counselor to underprivileged children at a summer camp in Glendale in 2008. In a statement, Camp Max Straus confirmed Holmes had worked there for eight weeks. The camp provided no other detail about Holmes but said such counselors are generally responsible for the care and guidance of roughly 10 children.
View photos from the scene of the shooting at an Aurora, Colorado theater
