Apart from the few idiots who just want a ruck, it's just threats designed to persuade some to give in to leavers. of course, the media like it as it gives them sensational headlines so in effect they can't really be relied on as a guide as to just how many would actually bother to turn up.It is impossible to hide from the quantity of comments online that say if brexit is cancelled there will be violence in the streets. Whether these people are keyboard warriors or serious remains to be seen. I sense the keyboard option.
What might happen is that there could be a boycott of the next election or referendum if there is a second one as a fir of pique. That's just an indicator of their lack of foresight because their dissatisfaction with things would then get no official recognition.