Is it normal for a democracy to prosecute secessionist leaders for rebellion?
They are not being prosecuted for being secessionist1, they are taken to court because they have been charged by breaking Spanish law. It is up to the judges to decide the issue (and of course, Puigdemont and others are entitled to due process, defense, and appeals up to the European Court of Human Rights).
that revolution is a basic right of the citizens of any democracy
No. No country, democratic or not, recognizes that right (at least for their own citizens). Of course, in some countries you have the right to call yourself "revolutionary" as long as you support the "revolutionary" government2.
Rights are granted by laws. A revolution is refusal to obey those laws; it is the creation of a new legal system not bound by the previous one, and aimed at replacing it. Few countries grant a right to secession and from those some seem unpractical (Liechenstein, Moldova) or specific to certain far away territories (France, Netherlands).
While I fully understand that a move to imprison those who might otherwise further a push for secession might be a very effective way of keeping a country together, at least in the short term, a leader forcefully taking over a region within his country, deposing its leaders and then taking them to court for fighting for freedom would seem to be something a dictator would do, not the leader of a democracy.
If we were talking about 65%, 70% of a region pushing for independence in a "do or go to jail/die" situation, the current intervention would be little effective. But that is not the situation:
numbers are not even close to that. To put the figures into context:
the two political parties pushing for independence got 39.6% and 8.2% (total 47.8%) in the last election.
turnout at the last referendum was stated by the Generalitat (without independent verification) to be of 43%. Compare to turnout at independence referendums in Scotland and Quebec (80%-90%).
turnout at a pro-independence referendum in 2014 (without police intervention) is estimated between 37% and 41% (https://en.wikipedia.org/wiki/Catalan_self-determination_referendum,_2014#Results). Again, no independent verification, and the Catalonian government did not issue turnout data
While it could improve -which government couldn't?- Spain is not the tyrannical government some claim it to be:
It is a democratic country with rule of law. If it were not, it could not be part of the EU.
It regularly ranks high in decentralization ranks. For example, it ranks higher than the USA3:
Regional government chosen by elections? Check.
Ability to enact and enforce its own laws, as long as they do not contradict the Constitution? Check.
Regional language official and taught at schools4? Check.
Regional police forces? Check.
Self-management of part of the taxes? Check.
So much for
a long train of abuses and usurpations, pursuing invariably the same Object evinces a design to reduce them under absolute Despotism.
And that leads to the absence of massive general strikes, public workers leaving their jobs en masse, and all the struggle that happens in actual revolutions/independence movements against actual oppressive regimes5. This also explains why independentist parties and organizations will take part in the next elections, instead of boycotting them.
taking them to court for fighting for freedom
These are not the charges.
would seem to be something a dictator would do, not the leader of a democracy.
See the point about Rule of Law. And it is not the the President who has filled charges against Puigdemont and others, and certainly it will not be him who will proceed with the trial and judge them.
And, last but not least:
a leader forcefully taking over a region within his country,
What has happened is that the regional government has been ousted and new elections have been called for (December 21st); once those happen Catalonia will regain its self-government.
1There have been (legal) independentist parties, propaganda and elected leaders since the current Constitution has been in place. Being independentist is not the issue.
2Mexico kind of aced this one, with its PRI or Partido Revolucionario Institucional(Institutional Revolutionary Party).
3Of course rankings of this type are complicated and you could alter some valorations so you could argue that the USA is higer, or not. The important point is that it is at the top of the list.
4To put an example, one of the most conflictive issues is the teaching of and in Spanish in Catalonian schools. The central government "imposition" is that 25% of the time lessons must be done in Spanish.
Do you imagine New Mexico issuing a law so that schools teach in Spanish and the Federal Government answering "well, as long as 25% of the time you teach in English we are ok with that"?
5Pro-independentists often quote the example of Kosovo independence, but neglect to mention the thousands of casualties that happened before that.