Charlie Sheen got heavily booed and people walked out of his show in Detroit tonight.

According to multiple people inside the Fox Theatre ... the majority of the audience heckled not only Charlie, but his buddy Simon Rex, who came out and started rapping.

We're told Charlie started the show with a Q&A session -- answering audience questions about everything from fave porn stars to crack cocaine (

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). Then Charlie played clips from a network interview he did and ripped it apart.

At one point, Sheen tried to deflect the angry crowd by pointing out that he had his own app ... and they didn't (see video above).

That went on for about 30 minutes ... and the crowd was initially cheering for Charlie. He even sent out a twitpic of the enthusiastic audience.

But then the crowd got ugly.

We're told Charlie called on Simon Rex for backup ... sending him on stage to rap -- and when that backfired they played the new track, "Winning" ... which Snoop Dogg recorded for Charlie.

We're told that got the crowd fired up ... but when neither Snoop nor Charlie came out on stage they started booing again.

At that point most of the crowd started walking out -- and the house lights were turned on.