From the wikipedia link:

>The Bangladeshi Declaration of Independence was broadcast from Chittagong by members of the Mukti Bahini—the national liberation army formed by Bengali military, paramilitary and civilians. The East Bengal Regiment and the East Pakistan Rifles played a crucial role in the resistance. Led by General M. A. G. Osmani and eleven sector commanders, the Bangladesh Forces waged a mass guerrilla war against the Pakistani military.

The Bangladeshi military organized the resistance. It was not a simple civilians-with-guns-rising-against-tyranny scenario.