10:05 AM PT -- Guns N' Roses just made the concert official for tonight at the Troubadour ... tickets are $10, so basically a giveaway. Fans can check out a ton of memorabilia and other GNR merch at the old Tower Records store where Slash used to work.
Guns N' Roses are about to rock the City of Angels with an impromptu concert ... and cops are assembling on the Sunset Strip for the mad dash to get a seat.
Law enforcement tells us they have a ton of Sheriff's deputies setting up around the old Tower Records ... in anticipation of the GNR announcement at 10 AM.
We're told there will be 250 tickets given away at noon for the concert -- first come, first serve.
Cops are expecting hundreds of fans to descend upon Tower Records when the announcement is made. It's still unknown when and where the actual performance will go down.
And get this ... we just got pics from the Troubadour and you can clearly see "GNR" on the equipment outside. Smells like a show to us.