After a Dhaka University professor asked a woman to remove her veil during his class, Muslim extremists called for his death, posting his personal details online along with tips on how to kill. Today, he remains under constant guard by armed police, stays mostly at home and bars his front door.

Azizur Rahman is among a growing number of political moderates and intellectuals seeking protection in Bangladesh, where at least 15 writers, activists, religious minorities and foreign aid workers have been killed in targeted attacks since the start of 2015. Islamist extremists have claimed responsibility for the killings, which have prompted some Bangladeshis to go into hiding, and others to seek asylum in the United States and Europe.

Some of the violence has taken place at Rahman's university in Dhaka, the Bangladeshi capital. Last year, secular writer and blogger Avijit Roy was hacked to death and his wife was critically injured in a savage attack on campus. In 2004, members of a banned Islamist group stabbed poet and linguist Humayun Azad at a campus book fair; he died months later.

Rahman, a psychology professor, pleaded with the government for 24-hour armed security outside his classes and at the modest campus apartment where he lives with his wife. Plainclothes policemen follow his every move around the sprawling, tree-filled open campus where he teaches five days a week.

Rahman said in an interview that he had asked the student to remove her veil because "I should know whom I am teaching. ... I told her if you show your face in the (student) identity card, you can also do so in class."

As the girl refused, the exchange was captured on a cellphone video by another student in the class. The next day, it had been uploaded onto the Facebook page of an Islamist group called Salauddiner Ghora, or The Horses of Salauddin — along with a clarion call for its followers to murder Rahman, whom they called "un-Islamic."

With the video, the group also posted Rahman's photograph, personal phone number and Facebook account. It also posted a link to an online blog detailing strategies for killing someone quietly, and links to two videos on YouTube on how to cut with a knife.

Rahman said he believes extremists have had their eye on him for some time, and are "capitalizing on the veil issue."

"Militants and persons belonging to religion-based organizations have carried out a hate campaign against me in a planned manner," he said.

Rahman has been critical of those who opposed Bangladesh's independence from Pakistan, won through a brutal war fought in 1971. The issue has exacerbated tensions between Prime Minister Sheikh Hasina's secular government and the country's Islamist opposition since the government set up special tribunal proceedings to prosecute 1971 war crimes. Though some rights groups and foreign governments have called the proceedings flawed, the trials have continued to convict opposition leaders and sentence them to death.

On Thursday, the Supreme Court rejected a petition from the leader of the Jamaat-e-Islami opposition party, Motiur Rahman Nizami, seeking a review of his war-crimes conviction and death sentence. Amnesty International urged Bangladesh to stay Nizami's execution and avoid stoking tensions, noting the court's decision had already triggered countrywide protests. "Taking another life will just perpetuate the cycle of violence," the group's South Asia campaign director, Jameen Kaur, said in a statement.

Rahman is not the only Dhaka University teacher to receive death threats, Vice Chancellor A.A.M.S. Arefin Siddique said.

"Threats from unnamed senders are nothing new, but an open declaration to kill the professor is a precise one," Siddique said. He has urged the city's police chief to be proactive in the case by going after those posting threats online.

"The persons calling for murder should be arrested," Siddique said. He added that university officials "are doing everything to ensure (Rahman's) security and safety."

The Bangladeshi government insists it is working to stop the attacks, but so far it has charged no one in any of the 15 killings since 2015.

Meanwhile, the brutality continues, usually by a handful of young men wielding knives or cleavers to hack their targets to death. On April 30, a Hindu tailor was murdered. The government has advised people at risk to simply lay low and try not to offend anyone.

The government says the political opposition orchestrated the attacks to stir chaos, though the opposition denies it and says it is being scapegoated. The government denies any involvement by transnational jihadist groups, though all 15 attacks have been claimed by the Islamic State group or various al-Qaida affiliates in Bangladesh.

This week Singapore announced that it had detained eight Bangladeshi workers suspected of planning attacks linked to the Islamic State group in their home country, and Bangladesh detained another five who had been deported back from Singapore for suspected IS ties.

Officer Abu Bakar Siddique, who heads the police station that registered Rahman's complaint, said he had no information about the group that had posted the threatening Facebook post, which had also praised the fatal stabbings on April 25 of two gay-rights activists, including an employee of the U.S. Agency for International Development.

This week, the Facebook account appeared to have been blocked or deactivated and was not accessible.

Among those coming to Rahman's defense is the student who refused to remove her veil.

Taposhi Rabeya, a third-year student, said she was surprised by the threats made against her professor, and that the video posted online did not fairly capture the full exchange Rahman had with his students. She said had worked out an understanding with Rahman that allowed her to continue wearing the veil in class.

"I hope all will realize the truth and uphold the dignity of the teacher, and refrain from further misinterpreting the issue," she wrote.