Lopez described a chaotic scene in the San Fernando Valley store among shoppers looking for video games soon after the sale began.
"I heard screaming and I heard yelling," said Lopez, 18. "Moments later, my throat stung. I was coughing really bad and watering up."
Authorities are searching for the woman accused of pepper-spraying other shoppers so that she could grab more discounted merchandise.
Twenty customers, including children, were hurt in the 10:10 p.m. incident, officials said. Shoppers complained of minor skin and eye irritation and sore throats.
"This was customer-versus-customer 'shopping rage,'" said Los Angeles Police Lt. Abel Parga.
The woman used the spray in more than one area of the Wal-Mart "to gain preferred access to a variety of locations in the store," said Los Angeles Fire Capt. James Carson.
"She was competitive shopping," he said.
Lopez said customers were already in the store when a whistle signaled the start of the Black Friday sale at 10 p.m., sending shoppers hurtling in search of deeply discounted items.
Lopez said that by the time he arrived at the video games, the display had been torn down. Employees attempted to hold back the scrum of shoppers and pick up merchandise even as customers trampled the video games and DVDs strewn on the floor.
"It was absolutely crazy," he said.
Another customer said screams erupted after about 100 people waiting in line to snag Xbox gaming consoles and Wii video games got into a shoving match.
Alejandra Seminario, 24, said she was waiting in line to grab some toys at the store around 9:55 p.m. when people the next aisle over started shouting and ripping at the plastic wrap encasing gaming consoles, which was supposed to be opened at 10 p.m.
"People started screaming, pulling and pushing each other, and then the whole area filled up with pepper spray," Seminario said. "I guess what triggered it was people started pulling the plastic off the pallets and then shoving and bombarding the display of games. It started with people pushing and screaming because they were getting shoved onto the boxes."
The pepper spray wafted through the air, Seminario said, and she breathed some in and started coughing. Her face also started itching.
"I did not want to get involved. I was too scared. I just stayed in the toy aisle," she said.
By the time she and her husband, 27-year-old Cesar Seminario, got to the cash register 20 minutes later with a Wii gaming console and some Barbie dolls, the air was still smelling of pepper spray, she said.
Wal-Mart employees were taking statements near the front of the store from about eight customers who had been pepper-sprayed, Seminario said. "After we paid, we saw five that were in really bad shape," she said. "They had been sprayed in the face, it looked like, and they had swelling of the face, really extreme swelling of face, redness, coughing."
Nakeasha Contreras, 20, said she arrived at midnight and hadn't heard what happened. Even if she had, she said, she wouldn't have minded: "I don't care. I'm still getting my TV. I've never seen Wal-Mart so crazy, but I guess it could have been worse."
Joseph Poulose, who said he was hit with the spray near the DVD and video games display, criticized the store for failing to control the crowds.
"There were way too many people in a building that size. Every aisle was full," he said. Customers were stomping on photo frames and other items on the floor, said Poulose, who tried to protect his pregnant wife from the throng of shoppers inside.
"It was definitely the worst Black Friday I've ever experienced," he said.