The head of one of Congo's largest human rights groups has been found dead, and the United Nations' top human rights official said Thursday the man had suffered a pattern of intimidation because of his work.

The body of Floribert Chebeya Bahizire of Voix des Sans Voix, or Voice of the Voiceless, was found in his car Wednesday in a suburb of Congo's capital.

"For more than 20 years, Chebeya Bahizire had survived many death threats, arrests, and ill treatment due to his work as a human rights defender. He believed in the cause of human rights and was not afraid to pursue it against all odds," Navi Pillay, the UN high commissioner for human rights, said Thursday.

Amnesty International called on the government to launch an investigation into Chebeya's death.

"We are stunned and appalled by the suspicious death of such a prominent and respected human rights defender," said Veronique Aubert, deputy director of the group's Africa Program. "Floribert has been arrested and harassed by the authorities in the past. It seems he may have paid the ultimate price for his valuable work."

The rights group said Chebeya often complained he was followed and under surveillance by security forces. The organization said he was last heard from Tuesday night, when he sent a text to a family member that he had just met with a senior police official and was headed home. Passersby later found his body.

Interior Minister Adolphe Lumanu issued a statement Wednesday night ordering security forces to look into the death and offering condolences to the family.

For the past two decades, Voix des Sans Voix has worked to document human rights abuses across Congo, focusing on corruption in the military and foreign support for militias, according to the UN.

Amnesty and the UN said they were concerned about a growing trend of harassment of human rights activists in Congo.