EXCLUSIVE

R&B singer The Weeknd was arrested Saturday in Las Vegas ... after he was accused of punching a cop.

Law enforcement sources tell us a fight allegedly involving Weeknd started inside the Cromwell Hotel around 4:20 AM ... and when they arrived, the fight was still raging.

We're told cops attempted to break up the fight, and one of them pulled the singer into an elevator during the scuffle. That's when Weeknd allegedly punched him in the head.

The singer -- best known for his duet "Love Me Harder" with Ariana Grande -- was instantly hauled off to jail for battery of a protected person.