What is the Islamic legitimacy of demonstrations in a Muslim country ruled by an oppressive government, and is it permissible to rebel against the ruler and clash with him?

It is not permissible to rebel against a ruler unless his blatant disbelief (kufr bawāḥ) is proven by clear evidence and acknowledged by the people of knowledge. Furthermore, rebellion against him must be in accordance with the greater good. If it leads to a greater harm than the intended benefit of his removal, then it is not permissible to rebel against him. This is in accordance with the principles: "Warding off harms takes precedence over acquiring benefits," "One commits the lesser of two harms," and "Achieving the higher of two benefits."