Mike Epps -- Cops Investigate Gunshot During Nightclub Fight

Someone fired a gun inside a Hollywood nightclub during a showdown between Mike Epps posse and other clubbers -- this according to law enforcement sources.

Cops with really big guns (see video above) -- rushed to Tru nightclub just after midnight, responding to a call that a shot was fired inside the club, where the "Next Friday" actor was celebrating his birthday.

According to law enforcement sources, a fight had broken out between Epps' camp and other clubgoers when someone pulled out a gun and fired a warning shot.

We're told everyone in the club ran for the doors, including Epps, who then fled the scene. No one was seriously injured ... but two women who were inside the club say they were trampled by panicked clubbers who raced for the exits.

Another witness who was inside the club tells us, "Everybody scattered, everybody started fighting and everything." The witness says Mike had just arrived to the party when the gun went off ... and at that point the actor jumped in his car and left.

Cops are currently investigating, and would like to speak with Epps to sort out what happened.

TMZ shot footage of Mike arriving to party minutes before the shooting -- and he told us his birthday wish ... a "big booty girl."