The incident happened Dec. 28 at about 11 p.m. on the 12000 block of Knott Street.
Two suspects possibly in their late teens walked into the store. One of them grabbed an 18-pack of beer. As they attempted to buy the beer, the clerk questioned the suspect's age.
One suspect then attacked the store clerk. Alex Ruiz, a customer in the store to get coffee, stepped in to help the clerk. The second suspect then attacked Ruiz.
Ruiz said he couldn't stand by and watch the clerk get beat up.
"The first suspect was kind of subdued on the ground. You can see from the video I just kind of had my knee over his back and was kind of holding him down," said Ruiz Friday at a Garden Grove police news conference.
"What was really bad for me was that I didn't know if this guy had a weapon. I couldn't exactly let him up, so as I was holding him and restraining him, I had to watch the other assailant just take his time with the guy. It was pretty bad.
"This guy's going to come for me next, eventually -- I'm in front of his exit. And then I see him turning and charge the door. And then I just got to go meet him, and then you can see from the video he kind of like caught me once, and then we tussled and went to the floor, and then it kind of got really floppy from there, but then I pushed him off and he ran away," said Ruiz.
Ruiz suffered a black eye. The store clerk was not seriously injured.
The first suspect was described as about 17 to 20 years old, standing 5 feet 6 inches tall, weighing about 140 pounds, wearing a black hooded sweatshirt, black shorts and a black hat.
The second suspect was described as between 17 and 20 years old with dark hair, standing about 5 feet 7 inches tall, weighing about 150 pounds, wearing a gray hooded sweatshirt and blue jeans.
Both suspects were last seen running southbound on Knott.
If you recognize the suspects, call the Garden Grove Police Dept. at (714) 741-5704.
Alex Ruiz was awarded the Garden Grove Police Chief's Coin for Merit to recognize his good deed.
About a month ago, Ruiz, who works in computers, was at a convenience store in El Segundo when he stopped some teenagers from stealing beer.