The incident occurred near the intersection of Sixth Avenue and Market Street during the monthly "Critical Mass" bicycle ride through the city.Photographer Scott Allen said he was in the crowd when the incident occurred. He said a screwdriver came flying down apparently from a nearby apartment balcony and hit the officer in the head. San Diego police told 10News the screwdriver was sharpened to be like a knife.The officer was not seriously injured. He was taken to a hospital to be treated and received some stitches for his injury."Critical Mass" occurs during the last Friday of every month to raise awareness about bicycle safety in San Diego.There have been problems recently. On a ride in early July, a woman in her 50s cracked her chin after being pushed to the ground during a ride on the Mission Beach Boardwalk. After that, the woman's husband got into a fight with a bicyclist.However, Friday's incident appears to have happened coincidentally as the riders were making their way through the Gaslamp area.