by prominent pro- conservative Ian Miles Cheong claims to show the verbal altercation that led up to the shooting.
A man wearing a "Black Guns Matter" shirt urges a tattooed man wearing a camouflage bucket hat to mace him.
"Mace me, f**kin' mace me brother!" the man yells. "Walk up to me and f***ing mace me n****, mace me mother f****r!"
A woman can be heard in the background asking the man why he's using a racial slur as he continues to shout at the counter-protester.
The bucket hat man clutches what appears to be a can of mace as the two continue to exchange obscenities.
"F**k around and find out!" the man with the Black Guns Matter shirt screams before turning his attention to another man holding a baton.
Moments later, the victim is seen dropping to the floor behind the arguing men after the hiss of an aerosol can and a loud bang.
Two guns were recovered at the scene, and the suspect in custody is being held at police headquarters, DPD said.
A canister of Mace was also recovered at the scene, and detectives are still probing whether it was used in the attack, the police chief said.
The security guard still had his handgun in his clutches, and as cops swarmed him with their guns pointed, he dropped to his knees, raised his hands above his head and released his weapon.
He was arrested at the scene along with the second person, who was later believed to have no connection to the shooting.
Local businesses were warned of the potential for escalating violence early Saturday and were told to report seeing protesters wearing helmets, shields, or gas masks ahead of the rallies, KMGH reported.
Dramatic moment Denver protester is shot at rally as cops arrest two