73-year-old leader of the Bangladesh's largest Islamist party had refused to seek presidential clemency.

Nizami was hanged at Dhaka Central Jail at 12 midnight, a police officer who witnessed the hanging told media.

"The execution has been completed," he said, adding that a civil surgeon declared him dead after the body was kept hanging for over 20 minutes.

Dhaka's district magistrate, the inspector general of prisons and senior police officials witnessed the hanging inside the high security prison compound.

Nizami's final appeal against his death sentence for war crimes committed during the 1971 Liberation War against Pakistan was rejected by the apex court on May 5.

Elite anti-crime Rapid Action Battalion joined police to step up security around the Dhaka Central Jail in old part of the capital. The jail guards have beefed up the security inside the facility. The street in front of the jail has also been barricaded and closed to traffic.

Nizami's family members, including his wife, two sons and their wives, went to the jail earlier this evening after prison officials asked them to go and meet him for the last time.

Prison officials earlier said jail doctors checked the heath of the death row convict last night after the Supreme Court verdict was read out to him while senior jail officials saw him at his solitary confinement earlier today.

A former minister in ex-premier Khaleda Zia's BNP-led four-party coalition government, Nizami was in jail since 2010, when he was arrested to be tried 1971 war crimes.

He was given capital punishment in October 2014 by the tribunal after being convicted of "superior responsibility" as the chief of the infamous Al-Badr militia forces in 1971.

He was particularly found guilty of systematic killings of over 450 people alone in his own village.

With his execution, Nizami becomes the fifth top perpetrator to be hanged for the war crimes against humanity since the trial process began six years ago.

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