(Adds comments from U.N. investigators, rights watchdog) GENEVA, June 26 (Reuters) - The United Nations' top human rights official said on Thursday that political violence in Zimbabwe had corrupted the electoral process and created a "perversion of democracy".

U.N. High Commissioner for Human Rights Louise Arbour said serious human rights abuses by members of President Robert Mugabe's ruling ZANU-PF party, and in some cases by the opposition MDC, "are unacceptable and need to stop immediately".

"Victims and their relatives deserve justice. Those who perpetrate crimes must be held to account," the former U.N. war crimes prosecutor said in a statement released in Geneva.

Nearly 40 U.N. human rights investigators called on the 84-year-old Mugabe to heed African calls to postpone Friday's presidential election, following deadly attacks on his political opponents that caused Movement for Democratic Change leader Morgan Tsvangirai to withdraw.

Mugabe, who is bound to extend his 28-year rule in the one-candidate election, has rejected such calls and said there could be no interference in his country, even from the African Union.

"We are of the view that no election should take place in the absence of conditions that would guarantee the free, full and equal participation of all citizens in the electoral process," the independent U.N. experts said in a joint statement released in Geneva during their annual week-long meeting.

"We strongly urge the government of Zimbabwe to ensure respect for human rights and to abide by democratic principles and practices, in accordance with Zimbabwe's own domestic law and international human rights standards," they said.

Separately, the International Federation for Human Rights urged the U.N. Human Rights Council to hold a special session on "gross and systematic human rights violations" in Zimbabwe, including killings, torture, arbitrary arrests and intimidation. The Paris-based watchdog said the Council should launch an international commission of inquiry into violations carried out during the electoral process.

Jean Ziegler, a former U.N. special rapporteur on the right to food, urged Mugabe to leave office and retire to his farm.

"It is always painful to discover perversion in someone you've admired. I was in Harare at the liberation and I have a magnificent memory of Mugabe as somebody with the stature of (Congo's independence leader) Patrice Lumumba," Ziegler told the Swiss weekly L'Hebdo published on Thursday.

"But I'm obliged to state today that he seems to have gone mad." (Reporting by Laura MacInnis and Stephanie Nebehay; Editing by Caroline Drees)

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