Nigeria's Sokoto state has declared an immediate 24-hour curfew to quell protests demanding the release of suspects in the killing of Deborah Samuel last week, a statement from the governor's office said.

On Thursday, Samuel was beaten and burned by fellow students for alleged blasphemous statements about the Prophet Mohammad in a Whatsapp group. Two suspects were arrested.

The curfew applies to the state capital Sokoto. Nigeria is almost evenly divided between the largely Christian south and mainly Muslim north, where some states have adopted strict sharia laws, including death sentences for blasphemy.

A city resident witness described the protesters as Muslim youths.

"They first went to the Sultan's palace and started protesting, security forces fired teargas and live bullets in the air," the resident said.

"The protest then sparked to other areas of Sokoto metropolis such as old market, Hajiya Halima road, Aliyu Jodi road, Gidan Dare road."

President Muhammadu Buhari has condemned the killing and said there should be an impartial investigation. Nigeria's largest grouping of Christian churches has demanded the authorities to bring the culprits to justice.