The UN High Commissioner for Human Rights has called on Iraq to halt all executions and commute the sentences of hundreds of people sentenced to death.
A spokesman for Navi Pillay said the executions on Tuesday and Wednesday of 42 people convicted of terrorism charges was "obscene and inhumane".
The claim that the death penalty helped deter terrorism was a "fallacy", given the surging violence in Iraq, he added.
The number of people executed in Iraq rose from 18 in 2010 to 123 in 2012.
So far this year, the authorities have carried out 140 executions, according to figures compiled by the UN Assistance Mission in Iraq.
Ms Pillay's spokesman, Rupert Colville, said in a statement that large-scale executions of the sort reported by the Iraqi ministry of justice on Thursday "are not only obscene and inhumane, they are most probably in contravention of international law".
"They are also undermining efforts to build a more stable, less violent society in Iraq. The mass execution carried out over the past two days is particularly perverse given that yesterday was World Day Against the Death Penalty," he added.
The Iraqi government maintains that it only executes individuals who have committed terrorist acts or other serious crimes against civilians.
"In our view, the claim that using the death penalty can help deter terrorism is clearly exposed as a fallacy, given the soaring casualty rate in Iraq, which has occurred over roughly the same period as the dramatic and shocking increase in the use of the death penalty," Mr Colville said.
The UN has said that 5,740 civilians have been killed in Iraq since January - almost double the figure it reported for the whole of 2010.