Bangladesh's war crimes tribunal has ordered the execution of a senior political leader after convicting him of atrocities during the country's 1971 independence war, triggering violence outside the court.

Three Molotov cocktails thrown by suspected anti-government activists exploded outside the courthouse in central Dhaka as Abdus Subhan, a vice president of one of Bangladesh's largest parties, Jamaat-Islami, was found guilty of murder, genocide and torture.

The verdict is expected to further inflame tensions in Bangladesh where an alliance of opposition parties, including Jamaat, is trying to topple the government of Prime Minister Sheikh Hasina.

At least 87 people have died since early January when the leader of the main opposition Bangladesh Nationalist Party (BNP) called on supporters to blockade roads, railways and waterways to force Hasina to call new polls.

Justice Obaidul Hassan, head of the International Crimes Tribunal (ICT), handed down the sentence, saying the 79-year-old leader would be "hanged by the neck until his death".

"He was found guilty of six out of nine charges. As a leader of Jamaat, he collaborated with Pakistan army and he carried out in the name of Islam," prosecutor Sultan Mahmud told reporters.

Subhan is the 17th person to be convicted by the court, which is a domestic tribunal set up by Hasina's government without any international oversight.

Prosecutors said Subhan was the head of Jamaat and a pro-Pakistani militia in the northwestern district town of Pabna and he actively took part in the murder of hundreds of innocent villagers and minority Hindus in the 1971 conflict when the then East Pakistan seceded from Islamabad.

Defence lawyers said they would appeal the verdict as the charges against Subhan were "false and baseless".

The war crimes court has mostly focused on the trials of the Jamaat leaders who opposed the break-up of Pakistan and saw the liberation war by Bengalis as a conspiracy by majority-Hindu India.

Previous death sentences handed down against Jamaat leaders, including its supreme and spiritual leaders, plunged Bangladesh into its deadliest unrest in 2013.