6:00 PM PT -- Future's concert is back on, but now it's at the House of Blues ... and only for the first 800 people.
3:55 PM PT -- Future's concert has now been cancelled.
Future is throwing a free concert at the Roxy in LA tonight and it's not off to a smooth start because the cops just showed up ... in riot gear.
We’re told fans started lining at 9 AM, and the show doesn’t start till midnight. The trouble started around 1 PM, when wristbands were passed out for the 500 capacity room ... everyone went wild.
Cops got out their riot guns when people started pouring onto Sunset Blvd ... smack in the middle of the Strip.
The concert is the last of Future's 4 free "Salute the Fans" shows.