Police arrested a 14-year-old boy Tuesday who they say sexually assaulted two women in West Oakland.

Oakland investigators said the boy turned himself in at Alameda County Juvenile Hall in San Leandro. The boy, whom The Chronicle is not naming because of his age, is being held at juvenile hall.

Police said the boy robbed a 28-year-old woman on the 1200 block of Market Street on Jan. 6. After he took the victim's wallet, police said, he forced her into an isolated area, where he sexually assaulted her.

The second victim, also a 28-year-old woman, was robbed and assaulted Thursday evening on the 600 block of 11th Street, less than half a mile from the first attack.

Authorities said the assailant approached the second victim on a bicycle and demanded her purse before he assaulted her.

Police said the boy had simulated a gun in the attacks, but said they had not yet determined whether he had, in fact, been armed.

Officer Johnna Watson, an Oakland police spokeswoman, said it was "highly unusual" for someone so young to be suspected of such violent crimes. Police had taken the unusual step of naming the juvenile suspect.

"We strongly believe that if we did not put this information out, in the interest of public safety, he would not have turned himself in," Watson said.