I'm talking about a peaceful demonstration against a corrupt government. The point is: It can be seen as divisive speech because there will be people against it.
The Buddha himself protested silently on at least two occasions in the Dhp commentary; both relate to his family, though. He never seems to have protested against the cruelty of kings, etc. and his actions seem always to have been in the form of teaching.
A protest is always a showing that you are against something. If you are truly against it, this is paṭigha, or aversion, and it is always unwholesome. In Buddhism we look at the bigger picture, and try to see that samsara as a whole is meaningless. As such, we try to teach all people to give up their unwholesomeness - this means teaching all rather than taking sides. The Buddha was exemplary in this regard, teaching even the basest of human beings and never challenging evil directly.
If the protest is instructional - i.e. a means of reminding certain parties of the error of their ways, then it seems to be reasonable, provided one is an involved party (e.g. a relative or intimate to one party in a conflict).
Finally, one might argue a civic responsibility for lay people - e.g. voting, or even demonstrating simply as a duty (e.g. H. D. Thoreau's Civil Disobedience). As long as one simply sees it as a duty, this seems reasonable.
Some monks have, occasionally, protested against a government (for example, "this sermon was extremely critical of the Prime Minister").
The word "divisive" reminds me of rules against creating a "Schism in the Sangha" ... maybe the rule against "divisive" speech means, especially, speech which would divide the sangha (not the body politic).
Even in the Vinaya there are rules for orderly ways to express disagreement or to assert your belief.
I don't want to say that it's always correct (maybe it varies depending on the country etc.), but I also don't want to say that it's never correct!
It might depend on your motive too. I have met one (lay) lady who told me that she joins the front row of street manifestations, because (she said) her being in the front row will decrease the likelihood of violent clashes between protesters and police.
Vietnamese Buddhist monks were against the repressive rule of President Diem, a Catholic in South Vietnam in the early sixties. They conducted street demonstrations and were beaten and jailed by the military and secret police thugs of the Diem gov't. Finally one of their monks in protest burnt himself with gasoline in downtown Saigon. A photographer was present and these photos of a monk burning were instrumental in bringing down Diem's repressive gov't and also in providing support to anti-war protesters around the world. The protests by Buddhist monks in a predominantly Buddhist country culminated in the ousting of President Diem by the CIA (via his assassination by military personnel loyal to coup plotters) in September? of 1963. You can read more about this monk at wiki page link: http://en.wikipedia.org/wiki/Th%C3%ADch_Qu%E1%BA%A3ng_%C4%90%E1%BB%A9c