The man was in the store's tool section and went unnoticed until he began grabbing several small saws, including one meant to cut sheet rock, and started using them on his arms.
"He cut both arms with hand saws down to the bone," said West Covina police Cpl. Rudy Lopez.
Officers received several 911 calls and arrived to find the man lying unconscious in the aisle in a pool of blood.
An off-duty paramedic from the Pasadena Fire Department had been shopping nearby and hurried to the scene.
"The officers had already found the man down, face down, blood all over the store, multiple aisles, and the whole store is in chaos," the paramedic, Art Hurtado, told KNBC-TV.
Hurtado thought the man was dead but when he checked he found breath and a slight pulse and said he thought to himself, "I can save this guy."
With help from police and store employees, Hurtado collected rope and rags from store shelves and put makeshift tourniquets on both arms, most likely saving the man's life, police said.
"Were we in a good place for it? No, but you improvise," Hurtado said. "If I didn't have rope I'd have used my shoelaces. We would have made it work."
The man was in surgery hours after the incident, said Lopez, who knew no more about his condition.
The man, who looked to be in his 40s, was carrying no identification, has been unable to answer questions, and was not heard saying anything in the store, so police do not know his name or why he cut himself, Lopez said.
Police interviewed about five people who said they saw what happened, and Lopez said there were likely many more who quickly left the disturbing scene.
"It was pretty graphic," he said.