Mandatory voting: is it democratic, or opposite of it?

According to democratic ideology, the political life of the country is decided by the people. It is, if we translate it, “rule of the people”.
So, voting for a president is a democratic thing to do, or so I think.

But what if voting is mandatory by law, and all the citizens are forced to cast their vote, or else they receive punishment? (jail, fine, whatever)

Is that, like, super democratic, or completely the opposite? (authoritarian?)

What are your thoughts on making voting mandatory? Would you agree with that or not?