11:47 AM PT -- Unser was so drunk, he couldn't stand up straight and actually fell down an embankment during the arrest ... this according to the arrest report obtained by TMZ Sports.
The responding officer says he initially pulled Unser over when he saw the racing legend whiz by him in a black Volkswagen Beetle around 1:12 AM.
During the stop, the officer says Unser reeked of booze and couldn't keep his balance ... and the entire time Unser insisted he hadn't had anything to drink before getting behind the wheel.
At one point, the officer says .