Pro golfer Thorbjorn Olesen was arrested at Heathrow Airport in London on Monday after allegedly sexually assaulting a woman mid-flight ... and then PEEING in the aisle.
Officials say 29-year-old Olesen -- who placed 6th at the 2013 Masters -- was a drunken menace on his flight from Nashville to London ... screaming at passengers and acting like a moron.
The most serious allegation is the alleged molestation of a female passenger. The details surrounding that particular incident is unclear.
During the hellish flight, a witness told The Sun .