At least 47 students have been killed by a suicide bomber at a school assembly in the north-eastern Nigerian town of Potiskum, police have said.
The explosion at a boys' science and technical school in the town is believed to have been caused by a suicide bomber dressed as a student.
Militant group Boko Haram is believed to be behind the blast, police said.
The group has targeted schools during a deadly five-year insurgency campaign to establish an Islamic state.
Boko Haram - which translates as "Western education is forbidden", is waging a sustained campaign to prevent children from going to school.
It believes girls should not attend school and boys should only receive an Islamic education.
The explosion ripped through the assembly hall at the Government Science Secondary School, reports say.
Police spokesman Emmanuel Ojukwu told the BBC Hausa service the bombing had left 47 people dead and another 79 wounded.
"School children were at the assembly ground this morning at about 7:30 am [06:30 GMT], then a suicide bomber who was disguised and dressed as student detonated a bomb," a witness told the BBC.
"It was a devastating attack. Dozens of children lost their lives and many more were injured. Some had broken limbs."
One student told the BBC he saw the mutilated bodies of fellow students at the scene where emergency operations are ongoing. A resident reported seeing parents wailing at the sight of their children's bodies at the hospital.
Soldiers who attended the site of the explosion were met with fury by the assembled crowds who pelted them with stones and accused them of not doing enough to halt Boko Haram's insurgency.
Schools in Yobe state have been frequently attacked by Boko Haram militants.
The state is one of three in Nigeria that have been placed under a state of emergency as a result of the group's activities.
Potiskum, one of the largest towns in Yobe , has also been targeted before by Boko Haram.
Last week, a suicide bombing killed 15 people in the town.
The bomber joined a religious procession of the rival Shia Muslim sect, before blowing himself up.
Analysis: Will Ross, BBC Nigeria Correspondent
By setting off the bomb during the morning assembly, the aim was clearly to kill as many students as possible.
Few of the attacks here are ever claimed by any group but Boko Haram will once again be suspected. The jihadists have carried out particularly brutal attacks on schools before.
The word "Chibok" is now known in many parts of the world because of April's mass abduction of girls from that remote village. But there have been many other horrific attacks on schools which have received less attention - including last February's raid on Buni Yadi, in Yobe State.
Dozens of boys were burnt to death, shot or killed with knives in the dormitory. Female students were spared but told to never attend school again, go off and get married. Boko Haram wants the education of boys to be limited to strict Koranic studies only.
The insecurity in the north-east is so rampant, with entire towns and villages now in the jihadists hands, it will be extremely hard for other bombings to be prevented.
Who are Boko Haram?
- Founded in 2002
- Initially focused on opposing Western education - Boko Haram means "Western education is forbidden" in the Hausa language
- Launched military operations in 2009 to create Islamic state
- Thousands killed, mostly in north-eastern Nigeria - also attacked police and UN headquarters in capital, Abuja
- Some three million people affected
- Declared terrorist group by US in 2013
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