OAKLAND -- A 79-year-old man was pistol-whipped and robbed Thursday night outside his Dimond District home, police said Friday.

The man, who is a grandfather, was treated at a hospital for head injuries, police said. His name was not released.

The man was taking out the garbage, outside his home in the 3200 block of Sheffield Avenue, about 9:15 p.m. Thursday when the robber approached him, pulled a pistol and demanded money.

When the man briefly hesitated, the robber hit him in the head with the gun, and grabbed the man's wallet from his pocket before running to a waiting car that drove off, police said.

The robber was described as black, 30 to 35 years old, about 6 feet and 170 pounds with a medium complexion, wearing a black shirt, black jacket and black pants.

Investigators were trying to determine if the suspect is responsible for other robberies.

Police and Crime Stoppers of Oakland are offering up to $3,000 in reward money for information leading to the arrest of the robber. Anyone with information may call police at 510-238-3326 or Crime Stoppers at 510-777-8572.