Sarah Palin's oldest son allegedly broke into his parents' house and beat his dad bloody until cops showed up and busted him ... TMZ has learned.
According to an affidavit by the arresting officer, Sarah called police in Wasilla, Alaska Saturday saying her son, Track, was "freaking out" and on "some type of medication."
As cops were responding, they got an update that Track had broken through a window at the Palin house and was assaulting his father, Todd. Upon arriving, cops say they found Todd fleeing in a vehicle and bleeding from the face ... with Track still inside.
While trying to subdue him, the officer says Track called the cops "peasants" and told them to drop their guns. Eventually, they arrested him and got the full story.
According to Todd, he and Track got into a heated argument earlier in the night about picking up his son's truck. Todd told Track not to come since he'd been drinking and was on pain meds, but Track came anyway.
Todd told cops that he had armed himself with a pistol and was prepared to "protect his family." That's when Track busted through the window, disarmed his dad and started pounding on his head ... before Todd was able to escape.
As we reported ... Track was arrested on 3 separate charges related to domestic violence. It's the 2nd time in two years he's been busted for similar cases.