The man was arrested after the mother of a 10-year-old student filed a rape complaint against him. (FILE)

Bangladeshi police have arrested a religious school's principal for allegedly raping at least a dozen children under his care, an official said Friday, sparking demonstrations by hundreds of people.

Al Amin, the head teacher and founder of the Baitul Huda Cadet Madrassa at Fatulla outside Dhaka, claimed innocence, saying he was "possessed by Satan".

Madrassas are religious institutions that provide everything from basic Koran-based education to graduate-level religious studies.

Elite police from the Rapid Action Battalion (RAB) arrested the teacher on Thursday after the mother of a 10-year-old student filed a rape complaint against him.

Local RAB chief Lieutenant Colonel Kazi Samser Uddin said the young girl watched a TV news report on serious sexual assault and told her mother that the seminary founder "did the same to her".

"The mother immediately rushed to us with the complaint. Then we arrested the principal," Uddin told AFP.

The teacher later admitted he forcibly had sexual intercourse with several of his minor students.

"We found he raped and sexually assaulted at least 12 girls in the madrassa," Uddin said.

Hundreds of local people protested in the town demanding justice and exemplary punishment of the principal, who is also an imam of a local mosque.

The police also last week arrested two high school teachers at a nearby town for allegedly raping 20 students.

Uddin said those teachers have been blackmailing and raping the victims and some of their mothers for the last four years.

Rights groups raised concerns about the spike in the number of rape and sexual assaults in the Muslim-majority South Asian nation.

The Manusher Jonno Foundation, a local rights group, published a report early this year saying 433 children were raped in 2018. Most were aged between seven and 12, it said.

Rights groups said "a culture of impunity" is partly to blame for what they say is a rise in sexual violence in the country.

According to Bangladesh Mahila Parishad, a women's rights group, only three percent of rape cases end in convictions.

The latest arrest came months after another madrassa head teacher was arrested over the April murder of Nusrat Jahan Rafi, 19, burned to death by attackers on the rooftop of her seminary.

The murder shook the nation of 165 million people after a police probe found that Rafi was murdered at the order of the madrassa principal after she refused to withdraw a sexual assault charge filed with the police against her seminary head.

Rafi's death received widespread media attention, and 16 men were arrested and are being tried for the killing. They face the death penalty.