The U.N.'s human rights chief is calling Saudi Arabia's mass execution of 37 men, including three who were sentenced as minors, "shocking" and "abhorrent."

Michelle Bachelet's office said Wednesday the beheadings in six cities across Saudi Arabia were carried out Tuesday despite repeated warnings from rights officials about lack of due process.

The men mostly belonged to the minority Shiite branch of Islam and had been convicted of terrorism-related crimes. The body and severed head of a convicted Sunni extremist were pinned to a pole as a public warning.

Bachelet said it was "particularly abhorrent that at least three of those killed were minors at the time of their sentencing."

She urged Saudi Arabia to review its counterterror legislation, expressly prohibit the death penalty for minors and halt pending executions.