Secession can certainly be illegal, but that simply means any outcome of an unsanctioned referendum is legally irrelevant. As far as I can tell, Spain is going

Secession can certainly be illegal, but that simply means any outcome of an unsanctioned referendum is legally irrelevant. As far as I can tell, Spain is going to great lengths to curtail the rights of citizens to express their opinions on secession. They're free to ignore the outcome of this vote, but raiding ISPs, shutting down political pamphlets, and reading private mail are not the actions of a democratic country—they're tactics straight out of an authoritarian regime. Secession is also illegal in the US (and we fought a war to prove it), but you don't see federal agents arresting people for starting ballot initiatives in California or shutting down Vermont secession websites.