We have a Spain-wide constitution. The Catalan government decided it has a democratic mandate by the people of its region to not abide by it. The NORMAL reaction by the state is to apply it, and stop the Catalan government from breaking it. It's only oppresive insofar laws and the judicial system are opressive.
Disclaimer: I'm a non-nationalist Spaniard living (but not born) in Catalonia. I don't particularly care about Catalan seceding or not (except for pragmatic concerns such as freedom of movement, etc.). I do care about whatever country I live in being able and willing to enforce its laws, insofar they are enacted by legitimate democratic institutions (and thus can be changed or repelled by democratic means) and there's freedom to disagree with them.
I also support a secession referendum for Catalonia. There's a very strong and majoritary demand for it that the state should address. But it needs to happen in due lawful process.