5:48 AM PT -- Law enforcement sources tell us as they were leaving, a member of Thug's crew ran into a police cruiser, prompting the vehicle search which uncovered the weapons.

Young Thug was busted by cops after they allegedly found the rapper and members of his crew in possession of multiple weapons.

Thug held a party at Dave & Buster's Hollywood Thursday for the release of his new album, "Slime Language," that also was serving as his 27th birthday party. In a video shot outside the venue, officers can be seen escorting the rapper in handcuffs to a waiting police cruiser.

YT was booked just after 3 AM for carrying a concealed weapon in a vehicle ... a felony.

He's currently being held on a $35,000 bond.