Abdul Quader Molla, 65, the fourth-highest leader of the Jamaat-e-Islami party, had been given a life sentence in February by Bangladesh's International Crimes Tribunal.

The tribunal has since January convicted six Islamists of crimes related to the 1971 war, in which pro-independence fighters battled Pakistani forces which were helped by local leaders.

Molla's life sentence had sparked deadly protests and widespread riots and there was fresh unrest on Tuesday as he was sentenced to hang, with Jamaat supporters torching vehicles in the southeastern port city of Chittagong.

"There were about 2,000 Jamaat protesters. They rioted, torching a police van and a private car," local police chief Mohammad Mohiuddin told, adding police fired rubber bullets and tear gas.

Molla was convicted of rape, murder and mass murder including the killing of more than 350 unarmed Bengali civilians, a poet and a top journalist during the war when he was a physics student at Dhaka University.

Prosecutors described him as the "Butcher of Mirpur", a Dhaka suburb where he committed most of the atrocities.

The conflict led to the creation of Bangladesh from what was then East Pakistan.

Defence lawyer Tajul Islam said: "We are stunned by the verdict. This is the first time in South Asian judicial history that a trial court sentence has been enhanced by a Supreme Court."

Islam said the defence would seek a review of the verdict in their final attempt to avoid hanging, which the prosecution said could be carried out later this year once all legal appeals have been exhausted.