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If you're one of the 1 million-plus people planning on storming Area 51 later this year -- you might wanna rethink that ... 'cause it ain't gonna be a walk in the park.

Law enforcement sources in the Nevada area -- where the notorious U.S. Air Force base is located -- tell TMZ that all the agencies that need to be aware of a potential raid on the premises are briefed and prepared for folks who might wanna try something come September.

The viral Facebook event caught fire last week with more than a million RSVPers who say they're down to head out there on Sept. 20 ... in hopes of learning the truth about alleged captured aliens on the base.

We're told the right people are monitoring all this online chatter about a potential raid, and the way the powers that be see it -- anyone who tries to follow through will be met with serious obstacles.

First and foremost, we're told anyone who commits a crime in or around Area 51 -- including trespassing -- will be arrested and prosecuted to the fullest extent of local and military law. Our sources also tell us local cops will work together to curb any attempts to even wander near Area 51 property .. and they're prepared to use non-lethal force, such as tear gas and pepper spray.

There are also natural hurdles folks will have to overcome. The route to get to Area 51 follows a narrow two-lane highway about 3 hours outside of Las Vegas ... and the terrain is rough to say the least. It's not really suited to withstand tens of thousands of cars.

If cops and the landscape don't stop ya, the desert critters just might -- the terrain surrounding Area 51 is full of snakes, scorpions and other dangerous animals.