OAKLAND, Calif. -- Police Saturday were offering $10,000 for information leading to the arrest of a man suspected of robbing at least 10 adult bookstores in the East Bay.

Police said Friday that a masked gunman has robbed six adult bookstores in Oakland, three in Berkeley and one in El Cerrito on Wednesday.

The first shop was hit in December, while nine have been robbed since early March. No one has been hurt so far.

Investigators say the suspect is usually masked and holding a gun when he enters the store and demands cash from the register.

They say no movies, magazines or merchandise have been taken.

In one of the recent robberies caught on tape, the gunman is seen walking into a downtown Oakland adult bookstore, brandishing a handgun and threatening the clerks and other patrons.

The manager, who asked not to be identified, said it was the third such recent robbery of an adult bookstore in the city.

Police released no other details about the 6:46 p.m. robbery at Cory's Adult Superstore at 2408 Telegraph and no arrests had been made.

The manager said police recommended he get a gun.

"The officer suggested I should arm myself," he told KTVU. "It's getting tough out there. These types of robberies are becoming an everyday happening."