The detectives saw the armed suspect, who was wearing a mask and a hooded jacket, walk into Mercury Books at 7535 Clairemont Mesa Blvd. about 8:45 a.m., San Diego police Officer Dan Lasher said.
Surveillance cameras captured the crime as it happened. The video shows the man walking into the store. He shows the clerk a gun, demands cash from the register and then stuffs it into a bag.
The clerk keeps his cool and cooperates the entire time. When the suspect leaves, he then calls 911.
Detectives who were waiting nearby for the suspect to leave the business confronted him in what they determined was a safe location and arrested him, Lasher said. His name was not immediately available.
Lasher said cash and a handgun were recovered.
A security consultant who did not want to be identified told 10News he provides surveillance for the business and three others that have recently been robbed.
"Video surveillance is key in all types of transactions like this," he said. "Not only this location but there's other businesses that have security and surveillance around it as well, so sometimes you get other video evidence as well from other locations."
Police believe the suspect is also responsible for robbing several other adult bookstores around town, including the Mercury Books on Balboa Avenue several days ago.
They identified the suspect as 28-year-old Oscar Smith of El Cajon. He was booked into jail on eight counts of armed robbery.