The six men were sentenced to death in November 2006 for the killing of 13 policemen during clashes over a forced eviction in the Sudanese capital, Khartoum
A stay of execution was granted by the Supreme Court of Sudan in early December 2009, but it expired on 6 January.
Rupert Colville of the UN Office of the High Commissioner for Human Rights (OHCHR) strongly condemned these latest executions and reiterated that under international law, the application of the death penalty can only take place in a very strictly defined set of circumstances.
One of these, he told a news briefing in Geneva, is that the sentence of death can only be issued after a trial in which minimum standards for fair trial were respected.
Mr. Colville noted that after their conviction and sentencing, the Government's attention was drawn to various reports that the men had been detained without access to legal counsel for five months following their arrest, and that they confessed to murder under torture.
Also, just hours before the execution took place, the Secretary-General's Special Representative in Sudan sent an urgent letter to the Government requesting that no execution take place.
"Regrettably, this appeal was ignored," said Mr. Colville, adding that the latest executions highlight the growing number of executions that have taken place in Sudan in recent times, many of which were carried out after trials which failed to conform to international standards.