This was not sanctioned by the rest of Spain (represented by Madrid), because it directly opposed the Spanish Constitution -- and because, you know, no country is thrilled when a region wants to secede. Madrid told people not to vote, which led to a massive victory for the pro-independence side (but a very poor turnout), which meant that -- given that Madrid basically said 'We haven't agreed to this referendum, so it means nothing' -- when Puigdemont basically declared Catalonia independent, Madrid stripped Catalonia of its regional autonomy. The people who organised the referendum and declared Catalonia independent fled across Europe, and have been recently been sentenced in absentia.

I am coming a bit late I am afraid but I must say that actually that is NOT what happened, not even close. For a start, there is not just some residents of Catalonia invested into independence but the majority of them. And the most important thing, NO ONE declared Catalonia independent after the referendum. In fact the referendum was NOT allowed by the spanish goverment and that's actually the real reason why catalonian politics were pursued and imprisoned.

Truth is that spanish constitution actually allows regions to call for not binding referendums, which is what catalonian independence parties were actually doing at that time. But the spanish goverment knew very well what would be the result of it, allowing independence parties to gain a lot more power from people and momentum to continue their efforts to become independent and at some point in the future eventually ending into a declaration of independence. So because spanish goverment was really really scared of this future they panicked and tried to end this referendum at all costs. First they decided to directly declare the referendum illegal (I repeat, it was not) and tried to convince people of this. As this was not actually working but having the opposite effect they started to become more aggresive and scared. Finally, they ended finding a loophole that allowed them to call a very very ambiguous constitution article not intended for such thing about their claim the referendum was illegal. But because catalonian politicians decided to continue with it anyway they ended pursuing and imprisoning them.

So in the end this was actually one of those moments in history where a goverment changed and bended the law to its favour against someone that was actually fighting for a cause using legal ways.

Source: I am from Spain.