Bangladesh has boosted security in the capital hours before a special tribunal is to deliver a verdict against the top leader of an Islamist party on war crimes charges.

The charges facing Motiur Rahman Nizami, chief of the Jamaat-e-Islami party, stem from the nation's independence war against Pakistan in 1971.

The prosecution says Nizami acted as the supreme commander of a militia group that carried out a systematic plan to torture and execute pro-liberation supporters during the war.

The war is still an emotionally charged issue in Bangladesh and previous verdicts by the tribunal have sparked street violence. Ahead of Wednesday's verdict, police and paramilitary units were patrolling the capital.

Bangladesh says Pakistani soldiers, aided by local collaborators, killed 3 million people and raped 200,000 women during the nine-month war.