Ali al-Nimr was given the death penalty in May after taking part in demonstrations three years ago for democracy and equal rights in Saudi Arabia's oil-producing Eastern Province.
"Any judgment imposing the death penalty upon persons who were children at the time of the offense, and their execution, are incompatible with Saudi Arabia's international obligations," the U.N. group said in a statement Tuesday, invoking the U.N. Convention on the Rights of the Child, to which Saudi Arabia is a party.
France said it was "concerned" about the situation.
"Opposed to the death penalty in all cases and circumstances, we call for the execution to be called off," Foreign ministry spokesman Romain Nadal said.
France does not usually comment on death penalty cases in Saudi Arabia due to their frequency. It has nurtured strong relations with Riyadh Wahhabi due to its tough stance on the Shia Muslims.
Nimr was convicted of sedition, rioting, protesting and robbery in the Eastern Province district of Qatif, home to many of the Wahhabi-ruled kingdom's minority Shia, who say they face entrenched discrimination.
Nimr, who activists said was 17 at the time of his arrest in 2012, was also convicted of chanting anti-state slogans in illegal protests and inciting others to demonstrate, according to state media.
"Saudi Arabia's plans to behead and crucify someone arrested as a child are indefensible," said Donald Campbell, spokesman for international human rights charity Reprieve.
"The international community – particularly Saudi Arabia's close allies, the UK and the U.S. – must stand with the French government and U.N. experts against this outrage, and call on the Saudi authorities to put a halt to this unjustified killing."
The conviction of Nimr, a nephew of Sheikh Nimr al-Nimr, a prominent Shia cleric who is also on trial, followed that of Rida al-Rubh, 26, the son of another cleric who has been critical of the authorities.
The clerics are part of a group of around a dozen defendants on trial for their part in protests and violent unrest in Qatif, particularly in the village of Awamiya, where police officers and facilities have been attacked.
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