The UN's torture investigator has made an angry return to South Africa after being refused entry to Zimbabwe.
Manfred Nowak was detained by officials in Harare who said he had no clearance to visit, despite his insistence he had an invite from the prime minister.
"I have never in any other country been treated in such a manner," Mr Nowak, who had planned a week-long fact-finding mission, told the BBC.
He blamed his treatment on the divisions within the unity government.
"This is a major incident because you can't on the one hand invite a special rapporteur to meet the prime minister and on the other hand somebody gives an order to the immigration police not to let me in," he told the BBC's World Today programme.
His said his treatment showed there were clearly parts of the government who did not want him to assess "the current conditions of torture", and promised to file a strongly worded complaint.
Prime Minister Morgan Tsvangirai and his MDC party stopped co-operating with the unity government two weeks ago, accusing President Robert Mugabe's Zanu-PF party of failing to live up to its commitments in the power-sharing deal.
Allegations that MDC supporters and human rights defenders have been arrested, harassed and intimidated during the past few days highlight the urgency of objective fact-finding
Earlier this week, the MDC warned that Zanu-PF militia had launched a campaign of violence against it, reminiscent of last year's post-election violence.
Human rights group Amnesty International has warned the country is on the brink of sliding back into violence.
The BBC's southern Africa correspondent Karen Allen says this diplomatic snub reveals the tussle for power between two sides in an increasingly unhappy marriage.
Mr Nowak's original invitation to come and investigate allegations of torture and mistreatment came from Zimbabwe's justice minister, who is a member of Zanu-PF.
But it was withdrawn at the last minute, with officials citing a clash with a visit by a mediation team from the Southern African Development Community (Sadc).
Sadc is hosting a meeting in Harare to discuss the crisis.
Nevertheless Mr Tsvangirai intervened and authorised the UN investigator to proceed with his visit.
Earlier, the UN said in a statement that Mr Nowak welcomed regional efforts "to resolve the political crisis" in Zimabwe, but that the Sadc meeting was not a valid reason to cancel his visit.
"Recent allegations that MDC supporters and human rights defenders have been arrested, harassed and intimidated during the past few days highlight the urgency of objective fact-finding by an independent UN expert," the UN said.
Zanu-PF has described comments about allegations of violent attacks on MDC members as "cheap propaganda".