A citizen advocacy group that opposes marijuana proliferation, Smart Colorado, warned in a statement that public 4/20 celebrations "send a clear message to the rest of the nation and the world about what Colorado looks like."
"Does the behavior of the participants in these events reflect well on our state?" asked the head of Smart Colorado, Henny Lasley.
A smaller Sunday event scheduled at the park was canceled.
Saturday's attack recalled a similar shooting that left a police officer dead at a crowded jazz concert in Denver's City Park last summer.
The 22-year-old suspect in that case pleaded guilty to second-degree murder and faces at least 16 years in prison when he is sentenced at a hearing scheduled for June 21.
His attorneys said he was being pursued by gang members when he drew his weapon and fired.