Breaking the law - ever justifiable? If so, when?

Is it ever justifiable to break the law when campaigning? This is a topical question given for example recent campaigns against climate change/flying at Heathrow.

Few people, if any, would argue that law breaking is never justifiable - think of prominent examples of law breaking to achieve positive social change like Vaclav Havel and the 'Velvet Revolution' in 1989, perhaps inspired by people like Mahatma Gandhi to gain independence in India and Martin Luther King Jr campaigning for civil rights in the USA.

I do belong to a radical party that has this core value: 'Electoral politics is only one way to achieve change in society, and we will use a variety of methods to help effect change, providing those methods do not conflict with our other core principles.'
http://policy.greenparty.org.uk/values.html

It is justifiable to break the law when campaigning and in fact some may feel compelled or duty-bound to do so, often inspired by people like Gandhi (who in turn was influenced by Henry David Thoreau) . However, if the law is broken it must, in my view, generally: appeal directly to the sense of justice of the majority; not reject the rule of law; be non-violent; accept lawful punishment that results; be a shrewd tactical move (why do it otherwise?); be consistent with core green values.