A man told Evanston police he saw what he thought was an abandoned bicycle Monday morning near Dodge Avenue and Oakton Streets.
He picked up the bike and started to ride away on it when he was confronted by a woman who demanded he give it back to her.
He complied, but moments later a man and a teenager approached him in a car and shouted at him about having stolen the bike.
They got out and one hit him in the head with a hammer and struck him with his fists several times.
Police describe the adult assailant as a bald, male Hispanic in his 30s, about 6-feet tall and weighing 200 pounds, who was driving a grey Chevrolet.