Palestinian police have arrested 12 men after an attack on a UN-run school in Gaza that killed one man and injured several others.
One person was killed and several injured in Sunday's attack
The group that claimed it carried out Sunday's attack said the school in the refugee camp in Rafah was not in keeping with Islam.
UN chief Ban Ki-moon said he was "deeply disturbed" by the attack.
Correspondents say this is the latest in a series of crimes against those seen as defying Islamic teachings.
The attack took place on a sports activity day at the school, near Gaza's border with Egypt.
In a statement, the Islamist Salafi group said it objected to the idea of boys and girls mixing together and popular music being played.
It also threatened further attacks on sports camps for children for similar reasons.
The BBC's Aleem Maqbool in Ramallah says dozens of internet cafes, music shops and Christian bookshops have already been attacked in the name of religion.
The police say the arrests show that they are dealing with the problem.
But our correspondent says the reality is that a lot more will need to be done, and done urgently, to prevent Gaza sinking deeper into lawlessness and extremism.
The man killed was said to be a local politician of the Fatah party led by President Mahmoud Abbas.
The top UN official in Gaza, John Ging, was at the school at the time.
He said: "It is a major problem in terms of the breakdown of law and order."
Mr Ban urged the Palestinian government to take all measures to protect the UN Relief and Works Agency, in whose compound the school is run.