Pro-gun groups in turn have pushed back, accusing New York Mayor Michael R. Bloomberg's Mayors Against Illegal Guns, which is organizing the event as part of its "No More Names" national tour, of politicizing the July 20 tragedy that left 12 dead and 58 injured.
The Mayors Against Illegal Guns rally is set Friday at Cherry Creek State Park in Aurora, and Rocky Mountain Gun Owners is holding a "peaceful 'counter-rally' at the same park.
Colorado Ceasefire compared the counter-rally to the notorious funeral protests conducted by the Westboro Baptist Church.
"Shame on them. ... Is it appropriate to stage a rally to counter a vigil?" said Colorado Ceasefire in a Facebook post.
RMGO spokeswoman Danielle Thompson said the mayors' group is using the anniversary to promote gun-control legislation, noting that MAIG has actively pushed for firearms restrictions in Colorado.
"They're politicizing this — Mayors Against Illegal Guns is using this anniversary as a podium," Ms. Thompson said. "We don't intend to hold signs; we're simply attempting to show that there are two sides to this debate."
The MAIG event starts with a press conference at noon Friday, followed by the reading of "the names of gun-violence victims" that will extend until 12:38 a.m. Saturday, the moment the shooting began a year ago during the premiere of "The Dark Knight Rises" at the Century 16 theater.
"Help us send a message to the country that it's time to take action to reduce gun violence nationwide as we've done in Colorado," said Democratic state Rep. Rhonda Fields in a press release. "It's time to stop adding names to the list of gun violence victims."
The "No More Names" tour drew criticism last month when reports surfaced that among the names being read was that of Tamerlan Tsarnaev, the Boston Marathon bomber killed by police. A survey by the Washington Examiner found that roughly one in eight names were those of criminals shot by police or armed citizens in self-defense.
MAIG issued a mea culpa after Tsarnaev's name was read at a June rally in Concord, N.H., saying that "it should not have happened, and we sincerely apologize."
Colorado emerged as a hotbed of gun-related political strife in March when state Democrats passed three gun-control bills that later were signed into law by Democratic Gov. John Hickenlooper.
Two Democratic state senators — Angela Giron of Pueblo and John Morse of Colorado Springs — have been targeted for recalls based on their support of the firearms restrictions.
A Denver District Court judge is expected to rule Thursday on a challenge to the recall petitions.