Shibir is the student wing of the ruling coalition partner Jamaat-e-Islami.

The AI also urged Khaleda to bring to justice those who threatened him.

An AI letter, posted at its website on Tuesday, strongly condemned the threat to the retired Rajshahi University professor. It also expressed grave concern over the threat to writers, journalists and academicians' by Islamist groups for expressing secular views and advocating the separation of religion from politics.

Referring to the killing of two RU teachers for their secular views, the AI letter said, "The assailants are believed to be linked to Islamist groups. No one has been brought to justice for these attacks."

The letter, also sent to Law Minister Moudud Ahmed and State Minister for Home Md Lutfuzzaman Babar, asked the government to protect others who express secular views and advocate the separation of religion from politics.

AI said Shibir threatened Prof Hasan Azizul Huq after he spoke in defence of secularism and the separation of religion from politics at a seminar in Rajshahi on August 21. "The threats have been followed by anonymous telephone calls to Professor Huq's residence warning him that he would be killed."

Although Prof Hasan notified police about the threat against him and his family, they have not been provided with any protection and those known to have issued the death threats have yet to be brought to justice, AI added.

The AI demanded an immediate and thorough investigation into the incident, probe results be made public and those responsible be brought to justice.