The Jamaat-e-Islami party leader, Abdul Quader Mollah, was found guilty by a special war crimes tribunal in February and sentenced to life in prison. That sentence was appealed by both the defence and prosecution.
On Tuesday, a five-member panel headed by Chief Justice M. Muzammel Hossain ruled that Mollah be put to death for his role during the war. The panel found him guilty of ordering the killing of a family of four during a Pakistani army crackdown in Dhaka in March 1971.
Attorney General Mahbubey Alam said this was the final verdict and there was no option for another appeal. He said Mollah's family can seek presidential clemency.
Defence counsel Abdur Razzaq said they were "stunned" by the decision.
Hours after the verdict, Mollah's said it would enforce a 48-hour general strike beginning Wednesday morning across the country to denounce the ruling. Somoy TV station reported Jamaat-e-Islami's student wing activists torched a police car and smashed another car in southeastern city of Chittagong while protesting the verdict. No injuries were reported.
Mollah and his supporters say the case against him is politically motivated. Mollah's party is a key ally of the country's main opposition Bangladesh Nationalist Party headed by former Prime Minister Khaleda Zia, an archrival of current Prime Minister Sheikh Hasina.
Hasina formed the special tribunal in 2010 to try war crimes suspects. Bangladesh says Pakistani soldiers, aided by local collaborators, killed 3 million people and raped 200,000 women during the nine-month war.
Zia has accused the government of using war crimes trials as a guise to weaken the opposition. The government denies the allegation and says it rose to power in 2008 with the prosecution of war crimes suspects as one of its key election pledges.
Several other top leaders of Jamaat-e-Islami have since been convicted of similar charges.
The government says the trials are being held at an international standard, but New York-based Human Rights Watch has raised questions about the impartiality of the tribunal.
The earlier sentence against Mollah led to protests across the country by supporters as well as those who said the sentence was too lenient.
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