Animal rights activists recently terrorized a UCLA neurobiologist into abandoning his research on primates. Ethical concerns about animal experimentation are certainly legitimate, and I'd have a fair bit of sympathy if they were raised through legitimate democratic channels (e.g. public debate), but this is just deplorable. As Timothy Burke notes, the activists' behaviour displays "an anti-democratic arrogance born of unthoughtful righteousness."
Such hubris is what repels me from radical politics more generally. These dogmatists feel so assured of the infallibility of their moral opinions that they're willing to coercively impose them on others. This implies a startling disrespect for one's fellow citizens. If you're really in the moral right, then you ought to be able to persuade your fellow citizens of this, and hence get the needed reforms implemented through legitimate democratic processes. Hence, if you can't succeed democratically, perhaps it's just as well...
Alternatively, if you're not in a properly functioning liberal democracy, then your overriding goal should be to establish the necessary institutions. Any first order political objectives are of secondary importance, and should wait until they can pass the liberal-democratic test. (It'd plainly be disastrous were every self-righteous moralist to go about trying to coerce everyone else into following their dictates.)
Do these principles imply the impermissibility of civil disobedience? Perhaps. If our democratic institutions are properly responsive to reasons, then illegal forms of protest should be quite unnecessary. Though I guess some might worry that the antecedent condition is unrealistically utopian. (That would pose obvious problems for the principles in my previous paragraph.) Perhaps we could argue that civil disobedience is itself an accepted part of our democratic process, so long as the protestors take care to cause no harm, and submit to the subsequent legal punishment?
For another difficult test case: what if one lived in a society that overwhelmingly endorsed slavery? Would it be wrong to "illegally" help slaves break free? That might seem a tough bullet to bite, but I think there is some plausibility to the idea that - even then - one would do better to work through legitimate channels (if such exist). Changing public opinion would have more significant long-term effects than isolated lawbreaking in any case, so could be preferred even on fairly crude utilitarian grounds (so long as such efforts are sufficiently likely to succeed). And again, we need to factor in our own fallibility: it's not entirely obvious that in such a situation we would have sufficient epistemic justification for our anti-slavery beliefs to warrant coercive action on their basis.
There could be exceptions for extreme cases where the government is so corrupt that one owes no respect whatsoever to the laws (qua laws -- there might be some, e.g. against wanton murder, that you'd still respect on independent moral grounds). Outright revolution can be justified if it is the only hope for establishing the just liberal-democratic institutions required for peaceful reform. But when those institutions are already available, I think it's terribly irresponsible to agitate for revolution simply in order to impose one's preferred first-order political objectives. Disrupting civil society with such minor revolutionary acts as illegal activism is, I think, likewise unjustified.
I want to suggest that this principle holds no matter how righteous one's goals might be. This seems well-grounded on indirect utiliarian grounds, since it's otherwise apparent how, say, anti-abortion activists might take themselves to be warranted in bombing abortion clinics and the like. To prevent misguided beliefs from leading to atrocity, we need firm principles of political toleration. We cannot universalize a decision procedure that would allow us to act coercively whenever we believe it would do the most good. So we cannot rationally act in such a way ourselves; we must first subject our proposals to the same tests we would reasonably demand of others, and the liberal-democratic process seems the best option there.
As a procedural liberal, I endorse the claim that our first (political) loyalty must always be to the liberal-democratic process -- upholding, reforming, or establishing it, as appropriate. This loyalty may (and should!) be shared by all citizens, no matter their first-order political stripe. There is plenty of scope for legitimate activism within this process. But coercive violations of the democratic process, no matter how well-intentioned, should be met with universal condemnation.