A court has jailed two teachers in southern Bangladesh for making derogatory comments about Islam and the Prophet Muhammad, invoking a rare law from the British colonial era that makes insulting any religion a crime.

The case began when students at Hijla High School in Bagerhat district complained that the assistant teacher of a science class on Sunday dismissed the Quran as the word of Allah and said there was no heaven, Magistrate Anwar Parvez told the Associated Press late on Tuesday.

The students, aged 17 to 18, along with others from a nearby, Islamic school became incensed when the high school's head teacher backed up his colleague. A mob including students, parents and villagers attacked the teachers with sticks, forcing them to lock themselves in a room until police intervened, Parvez said.

"The situation went out of control," Parvez said, adding the mob "wanted to take law in their hands."

The magistrate of the quick-ruling court said the assistant teacher pleaded guilty to publicly insulting religion, and the two were sentenced to six months behind bars.

The law against insulting religion, imposed when Britain ruled the Indian Subcontinent, is rarely used and aimed at preventing communal clashes and inciting violence.

The Muslim-majority country - politically fractured between secularists and those wanting Islamic rule - has been roiled by an ongoing wave of deadly attacks on atheist writers, religious minorities and activists over the last two years.

On Monday night, a gang of young men stabbed two men to death in Dhaka, including the editor of a gay rights magazine who also worked for the US Agency for International Development.

It was the fifth such killing this year, after nine were cut down in 2015. International governments including the United States and aid groups have implored the Bangladeshi government to do more to safeguard free speech and protect members of civil society.