This wild display of a demonstration went down Monday at London's National Gallery -- which was housing the Diego Velázquez painting you see here ... getting absolutely crushed by a couple of Just Stop Oil advocates, who have a history of destroying art to make their point.

In this case, they were caught red-handed yet again ... only this time, instead of making a mess by splashing some crap on a painting -- they decided to just hammer away.

Watch ... this guy and gal -- who are proudly rocking their JSO shirts -- go to town on the glass cover of this Velázquez artwork, which is known as the Rokeby Venus. It doesn't look like they were able to get to the actual painting underneath, but they crushed the exterior.

London's Metro Police confirm the two activists were arrested on suspicion of criminal damage ... but there's more than just suspicion here, the evidence is clear and damning.

BTW, the painting already had some history of being vandalized. Back in 1914, suffragette Mary Richardson slashed the artwork of the "most beautiful woman in the world" for a) the nudity depicted by the male gaze and b) to make a political statement following the arrest of her compatriot, Emmeline Pankhurst .. who pushed for women to get the vote.

Remember, this same group has carved a rep for itself as art destroyers. They've gone around splashing paint and soup at other famous paintings there in Europe ... and each time, they're always wearing these same easily recognizable tees.

You'd think these museum security guards would wise up and stop letting them in.