A final appeal by the leader of Bangladesh's largest Islamist party against a death penalty for involvement in a 1971 liberation war has been rejected, his lawyers said, clearing the way for his hanging.

The Supreme Court on Thursday passed the order against Motiur Rahman Nizami, head of the banned Jamaat-e-Islami party, which opposed the war for independence from Pakistan.

The 73-year-old, who was also given life sentences for four other war crimes convictions, has exhausted all legal options and only a presidential pardon can now save him.

Jamaat called a nationwide strike for Sunday in a response published on its website.

The Supreme Court in January upheld the death penalty for Nizami on convictions of genocide, rape and orchestrating the massacre of top intellectuals during the war.

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The Jamaat leader, in jail since 2010, was originally handed the death sentence by a war crimes tribunal in 2014.

Bangladeshi authorities say about three million people were killed and more than 200,000 women raped during the conflict. The former East Pakistan broke away to become Bangladesh after the war.

The tribunal has sparked violence and drawn fierce criticism from opposition politicians, including Jamaat-e-Islami, who say it is victimising Prime Minister Sheikh Hasina's opponents.

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No Peace Without Justice, a rights group based in Italy, has called the tribunal "a weapon of politically influenced revenge whose real aim is to target the political opposition".

The government denies that.

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A senior Jamaat-e-Islami leader living outside Bangladesh told Al Jazeera in a reaction that the news of the verdict was "devastating".

"This is the not the first death sentence confirmed by the judicial system and quite a distinguished line of others are waiting after him," said the party official, who wished to remain anonymous.

"Not only that; at least 20,000 Jamaat workers are in prison without any charges.

"Police have raided and arrested many people attending religious or Quran classes. Even the international community rejects the allegations and also the court process."

'Politicised process'

International legal experts expressed concern over the lack of an appropriate accountability mechanism in Bangladesh and called on the United Nations to support an internationally supervised mechanism.

A joint statement, signed by six legal experts, said: "It is with deep regret that the current practice of the Bangladesh International Crimes Tribunal (BICT) is failing to uphold the highest international standards required in such cases, in particular where there is the imposition of the death penalty."

The experts, affiliated with a wide range of institutions, from the International Criminal Court to the US State Department, said that the central issue is that "any judicial mechanism, post conflict or otherwise, is about the pursuit of justice and accountability. It is not about revenge or political retribution."

"Sadly, the BICT has shown itself to be merely that, a politicised process that fails to uphold the very standards it was set up to address," the statement read.

The nation has seen a surge of violence in which atheist bloggers, academics, religious minorities and foreign aid workers have been killed.

In the last month alone, five people, including a university teacher, two gay activists and a Hindu, have been hacked to death by suspected armed groups.

The government has blamed that violence on Jamaat and other opposition groups. Jamaat denies any involvement.

Four opposition politicians, including three Jamaat leaders, have been executed since late 2013 after tribunal convictions.