Officers responded just after 12:30 p.m. Tuesday following a report of an attempted kidnapping on an Alameda-Contra Costa Transit bus platform at High Street and International Boulevard.
Family members told police that a person unknown to them threatened them and tried to take their 2-year-old, according to police.
The suspect was unsuccessful after several tries. Then the suspect assaulted the child's mother until a good Samaritan arrived and the suspect left the area, police said.
The suspect is described as a Hispanic man around 40 to 50 years old who is about 5 feet 10 inches tall and weighs an estimated 280 pounds, according to police, who said he was wearing black pants, a red and blue shirt and blue hat.
Anyone who recognizes the person is asked to call the Police Department's special victims unit at (510) 238-3641 or their tip line at (510) 238-7950.