I think that would be an easier thing to deal with. If "a few thousand ex military vehicles turned up (including a few working tanks) all armed to the teeth" turned up, it would be an armed revolt attacking a military base, so unlimited force could be used. Working tanks aren't allowed in private hands in the USA, but even if they were then ex military vehicles and a few working tanks, all manned by untrained or barely trained people, would be outmatched by current USA military with current equipment. The USA has a lot of military and all the kit money can buy.
A mass of unarmed civilians would be harder to deal with, IMO, because it would not be politically viable to use unlimited force against them even if there was the political will to do so. It would be possible to massacre thousands, tens of thousands of them, but it would cause havoc. At the very least, the government would fall. That's assuming that the soldiers would obey orders to massacre huge numbers of unarmed civilians, which is far from certain.
However, I'm pretty sure the US military has a lot of tear gas, a lot of soldiers to arrest people and a lot of vehicles to take the arrested people away. I'm also sure that if there is anything really secret in Area 51, it's not on the surface and won't be accessible without the right codes or an unfeasible amount of explosive. They could probably just close the secure parts and let the numpties wander around in the desert until they ran out of water and/or interest and had to leave, which wouldn't take long.
Or maybe they'd build a fake flying saucer and leave it in a shed on the surface for the numpties to freak out about, just for laughs.