Because of the worsening condition in Mindanao, the growing rebellion and other form of rebellion, President Ferdinand Marcos issued the presidential Decree No. 1801 which contains the declaration of Martial law in the country on September 21, 1972.
The imposition of martial law, coupled with the Government’s determination to disarm the populace, including the Moros, only strengthened the resolve of dissenting Muslims to struggle for independence. The first major armed defiance of the martial law regime occurred in Lanao Del Sur on October 21, 1972, when a rebel force of several hundred Moros attacked the Philippine Constabulary headquarters in Marawi City, seized the Mindanao State University campus and attempted to take the Pantar Bridge on the Marawi-Iligan road.

Sources: Peter G. Gowing, Muslim Filiponis; Heritage and Horizon