7/16 -- 9:40 AM PT -- Lil Reese is reacting to the charges with denial and in his own words, is "unbothered like an MF" ... and eagerly awaits his chance to vindicate himself.

Lil Reese is facing serious allegations -- he's accused of rape in Los Angeles, where police took him into custody ... TMZ Hip Hop has learned.

According to law enforcement sources, Lil Reese went on a date with a woman and following dinner, he and the woman went back to a Downtown L.A. hotel where Reese allegedly insisted they have sex.

Cops say the woman rebuffed Reese's advances, before he allegedly forced himself on her and raped her.

We're told the woman then went straight down to the hotel lobby, and called the police around 3:30 AM Sunday.

We're told officers promptly arrived on the scene and arrested Reese on suspicion of sexual assault and rape.

Boosie Badazz was an apparent witness to the arrest and put out an APB on Reese's friends to bail him out.

Reese was later released on $100K bond.

We reached out to his team ... still no word back yet.