With a line-up that includes Drew Barrymore, David Beckham, Orlando Bloom, and Ricky Martin, the UN's choice of ambassadors has been known to cause raised eyebrows or the odd smirk.

Seldom, however, has there been such anger, or questioning of the organisation's credibility, as that greeting the appointment of a new international envoy for tourism: Robert Mugabe.

Improbable as it seems, the Zimbabwean president, who is widely accused of ethnic cleansing, rigging elections, terrorising opposition, controlling media and presiding over a collapsed economy, has been endorsed as a champion of efforts to boost global holidaymaking.

Despite that fact Mugabe, 88, is under a travel ban, he has been honoured as a "leader for tourism" by the UN's World Tourism Organisation, along with his political ally, Zambian president Michael Sata, 75. The pair signed an agreement with UNWTO secretary general Taleb Rifai at their shared border at Victoria Falls on Tuesday.

Zimbabwe's state-owned Herald newspaper quoted Rifai urging tourists from around the world to visit : "I was told about the wonderful experience and the warm hospitality of this country … By coming here, it is recognition, an endorsement on the country that it is a safe destination."

The agreement will also see the two southern African countries co-host the UNWTO general assembly in August next year.

UNWTO said it had not appointed Mugabe to any formal position but acknowledged he would receive an open letter like other heads of state who have joined its leaders for tourism campaign.

The development has stunned human rights campaigners and political opponents, who regard Mugabe as a tyrant.

Kumbi Muchemwa, a spokesman for the Movement for Democratic Change (MDC), said: "I can't see any justification for the man being an 'ambassador'. An ambassador for what? The man has blood on his hands. Do they want tourists to see those bloody hands?"

Meanwhile, British MP Kate Hoey, chair of the all-party parliamentary group on Zimbabwe, said: "It is an absolute scandal – and an affront to the people of Zimbabwe, who didn't vote for Mugabe as their president but had him imposed because he used violence and the armed forces to hang onto power in defiance of the democratic will of the people of Zimbabwe.

"For a man who has destroyed his country's infrastructure and cynically engineered hunger to be an 'ambassador' for tourism is disgraceful – particularly as he has been personally responsible for the downward spiral of the economy and destroyed the hotel, travel and tourism industry in the process."

Mugabe and his allies are subject to EU and US sanctions preventing them from travelling to EU countries including Britain, although he does attend the UN general assembly in New York.

Muchemwa added: "Robert Mugabe is under international sanctions, so how do you have an international tourism ambassador who can't travel to other countries?

"The UN is losing credibility in this process. Does it think people should go to a country where the law is not obeyed? An MDC activist was murdered last Saturday. Zimbabwe is doing things which don't encourage the arrival of tourists."

There was also criticism from the Crisis in Zimbabwe Coalition, an umbrella organisation of civil society groups. Dewa Mavhinga, its regional information and advocacy officer, said: "It boggles the mind how the UN could appoint Mugabe as an ambassador of any sort. I don't think he's an appropriate person.

"It sends the wrong message to Mugabe that he is now acceptable to the international community. This is the same guy who last week was bashing gays and lesbians, who he says are worse than dogs."

Zimbabwe is rich in tourism potential, boasting attractions such as Victoria Falls, the ruins of Great Zimbabwe and the Hwange wildlife game reserve, and one of the best climates in the world. Its official tourism branding entices with "a world of wonders".

The past decade has hit the industry hard, with the national airline going bust, but there have been recent signs of a gradual recovery.

John Makumbe, a politics professor at the University of Zimbabwe, said of Mugabe's accolade: "I think it's ridiculous because Zimbabwe is one of the countries least used by tourists.

"Tourism is at its lowest level because of the political and economic crises it's gone through. Tourists really wish Victoria Falls was in another country, like South Africa.

"Robert Mugabe will do more damage to international tourism than good. His image is in tatters, his country is an international pariah.

"It undermines the reputation of the UNWTO as being detached from the reality on the ground in terms of human rights violations and political instability."

But after visiting the country last week, the UN high commissioner for human rights, Navi Pillay, urged western countries to lift their targeted sanctions, arguing that they are hurting the poor. She also called on Zimbabwe to pass reforms to avoid violence in the next election.

Mugabe's Zanu-PF party seized on his UN honour as evidence that opponents and media have exaggerated the country's problems. Spokesman Rugare Gumbo said: "There's no alternative but to accept the reality on the ground. We can theorise about sanctions but the reality is that the UN is in control of the situation. If you can't defeat them, join them: that is what we are witnessing."

He continued: "The situation on the ground in Zimbabwe is not as bad as portrayed. If we say this ourselves, you say it's propaganda.

"What do you expect from the MDC? They are paid by the US and Europeans and they have nothing else to offer. They keep making noise but the reality on the ground is different."

UNWTO, which has headquarters in Madrid, insisted that it had not awarded Mugabe an official title. Sandra Carvao, its co-ordinator of communications, said: "Correct would be to say UNWTO has presented both presidents with an open letter which calls for them to support tourism as a means to foster sustainable development in their countries to the benefit of their people and consequently ask them to support the sector in this respect."

She added: "UNWTO does not have an ambassadors programme and the receiving of the UNWTO/WTTC [World Travel and Tourism Council] open letter implies no legal commitment or title attribution to the country or the head of state or government in question."