Yes,it is legal and it is in the constitution. Martial law is one of the power of a president and it is his right to declare it on Marawi City where lawless criminals or terrorists strike fear,chaos and violence for the past months. That is his duty to preserve order and the lives of the innocent and law-abiding citizens. For me, martial law is implemented properly in Marawi City and President Rodrigo Duterte did a wonderful job of protecting his countrymen against insurgencies and terrorism made by well-funded criminals.