The most serious violence to take place was a series of bomb blasts in Riyadh in late 1966 and early 1967. The bombings, which caused no known casualties, were claimed by the North Yemen-based Nasserite organisation Union of the People of the Arabian Peninsula [ittihad sha'b al-jazira al-'arabiyya] (UPAP). After the attacks, Saudi authorities arrested several hundred Yemenis, executed 17 of them and expelled the rest.