Manfred Nowak was detained by officials in Harare who said he had no clearance to visit.
Though Manfred Nowak had an official invitation from the prime minister.
Manfred Nowak blamed his treatment on the divisions within the unity government.
Nation cannot invite a special investigator --- and somebody gives an order to the immigration police not to let him in,
Manfred Nowak said his treatment showed clearly parts of the government did not want him to assess "the current conditions of torture".
The trouble comes as the IMF announced that the unity government's "improved policies" were benefitting the economy.
"I have never in any other country been treated in such a manner," Mr Nowak, who had planned a week-long fact-finding mission.