Police launched a murder investigation after attending reports of fighting between two men in Bow, east London, at about 20:45 BST on Monday.
Officers believe the fight began on a 488 bus and continued on Parnell Road.
A man in his late 60s was found unconscious at the scene and later died in hospital.
A 73-year-old man was arrested on suspicion of causing grievous bodily harm and remains in police custody at an east London police station, where he is being questioned.
A 73-year-old man has been arrested after a fight that started on a bus led to another man's death.