As UN peacekeepers in Ivory Coast refused a demand by leader Laurent Gbagbo that they leave the country, Navi Pillay said in a statement that "in the past three days there has been more than 50 people killed, and over 200 injured."
She expressed concern about "the growing evidence of massive violations of human rights" in the restive west African country since Thursday and vowed "to ensure that perpetrators are held accountable for their actions."
Both incumbent leader Mr Gbagbo and his rival Alassane Ouattara claim to have won last month's presidential run-off election in Ivory Coast. While the latter has been recognised as the victor by the international community, Mr Gbagbo is clinging on to power.
Tension has reached boiling point in Ivory Coast's commercial capital Abidjan, where violence erupted Thursday during a protest march by Mr Ouattara's supporters, and where Mr Gbagbo's armed forces are in an uneasy stand-off with the UN.
"When people are victims of extrajudicial killings there must be an investigation, and there must be accountability," Miss Pillay noted. "However, the deteriorating security conditions in the country and the interference with freedom of movement of UN personnel have made it difficult to investigate the large number of human rights violations reported."
On Saturday, Mr Gbagbo ordered the 10,000-strong UN mission to leave the country, accusing it of arming rebels loyal to Mr Ouattara, but the UN chief dismissed the ultimatum and called on him to step down.
Miss Pillay said the UN mission in Ivory Coast "has received reports from hundreds of victims and members of their families about the abduction of individuals from their homes, especially at night, by unidentified armed individuals in military uniform accompanied by elements of the Defence and Security Forces or militia groups."
"Abducted persons are reportedly taken by force to illegal places of detention where they are held incommunicado and without charge. Some have been found dead in questionable circumstances," she added.
She urged all Ivorian parties concerned "to respect the human rights of all Ivorians, without discrimination".