How it actually happens and whether or not different commenters find it "legal" will not matter. If I deny your jurisdiction and have the military force to defend myself, your laws do not apply to me.
Sure, this is how some sovereign states formed in the first place. But even these were made "legal" under international jurisprudence.
Don't confuse exercise of power with legal aspects though. It can certainly be against a nation's laws to secede, and secede anyways if you can prevent the state from effectively exercising power (e.g. Algeria in the 1950s). But that didn't make it "legal", either under domestic or international law.
Even in international law if you want to secede you have to form a separate state, have it exercise effective control over a territory, and do so long enough to have "enough" other nations recognize that control. If you can do that then it doesn't matter what your parent nation's domestic laws were, but until you can do that it's shaky legal ground indeed.