Emergency at CBS Studios -- White Powder in Mailroom

Police and emergency vehicles arrived to CBS Television City in Hollywood moments ago ... after someone reported that white powder spilled out of a letter in the CBS mailroom.

Law enforcement sources confirm ... cops are investigating the scene.

Sources on the lot tell TMZ ... the letter was NOT addressed to anyone on "Dancing with the Stars" ... but so far, it's unclear who the intended recipient was.

It's not the first time CBS has had an anthrax scare -- just last year, the studio went on lockdown when production staff found white powder in a letter addressed to Bristol Palin.

Officials also intercepted a threatening letter addressed to "Late Late Show" host Craig Ferguson just last month. The letter also contained white powder -- which was later determined to be cornmeal.

3:21 PM:The L.A. City Hazardous Materials team is on the scene.