Police arrested a 36-year-old man after he allegedly assaulted a 73-year-old man at Ala Moana Beach Park on Tuesday.
The Honolulu Police Department and Emergency Medical Services said the suspect struck the victim on the head with an unspecified heavy object at about 7:10 a.m.
Paramedics treated the 73-year-old, who sustained a laceration, and took him to a hospital in serious condition.
Police said the victim and the alleged assailant are not known to one another.
Police arrested the suspect on suspicion of second-degree assault.