Preparations are in full swing at 1600 Pennsylvania Avenue for a doomsday scenario on Election Day -- fences and barriers are going up outside the President's residence, with dark days possibly around the corner.

As you can see, the White House is a total construction zone Monday -- on election eve -- as workers are busy putting up wooden and metal fences, and a big cement truck's there too.

It's another ominous sign as Election Day draws near ... things are already getting crazy across the country before votes are even counted.

As we reported ... police made a huge show of force over the weekend in Beverly Hills, where throngs of demonstrators gathered.

The boarded-up White House falls in line with what we've already seen in Washington D.C., where it seems the whole town is hunkering down and bracing for violence and unrest.