The United Nations torture investigator warned yesterday that Zimbabwe’s fragile coalition Government could collapse, saying that his expulsion from the nation showed that President Mugabe and his party continued to hold “the real power”.

Manfred Nowak, who was detained on Wednesday night when he arrived in Harare, blamed Mr Mugabe for blocking his entry and said there was evidence that torture was once again being used to stifle dissent. He said he would recommend that the UN Human Rights Council should take action against Zimbabwe.

Mr Nowak had been due to spend a week in Zimbabwe to investigate allegations of human rights abuses. He was originally invited by Patrick Chinamasa, the Justice Minister, in February, and then by Morgan Tsvangirai, the Prime Minister, but was told at Harare airport that he did not have security clearance.

The visit was supposed to signal a willingness by the new Government to address the claims of human rights abuses under the rule of Mr Mugabe. The President formed a unity government eight months ago with Mr Tsvangirai’s Movement for Democratic Change (MDC) but the Prime Minister has since refused to work with his adversary and has accused Mr Mugabe of reneging on his promises.

After Mr Novak returned to Johannesburg, he denounced his treatment as “a very alarming signal about the power structure of the present Government”.

He said: “It sheds a clear light as to where the real power is lying in this unity Government ... There are certainly some parts of the Government who do not want me to assess the current conditions of torture. There are strong indications that this was just not done by the Minister of Foreign Affairs without at least the knowledge of President Mugabe.”

He added: “There are alarming reports of violence in rural areas; of intimidation and torture.”

Ephraim Masawi, a spokesman for the President’s Zanu (PF) party, denied that Mr Mugabe was involved in the expulsion.

Mr Novak has joined a growing number of people expressing anxiety at a renewal of violence and intimidation against supporters of Mr Tsvangirai and human rights groups after a period of relative calm.

Amnesty International warned that “Zimbabwe is on the brink of sliding back into the post-election violence that marred the country last year”, when soldiers, police, intelligence agents and Zanu (PF) militiamen murdered at least 100 MDC supporters and tortured thousands to ensure that Mr Mugabe won the presidential run-off election. Mr Tsvangirai withdrew and Mr Mugabe was declared the winner of a one-man race that was dismissed overwhelmingly as a fraud.

A week ago, a group of heavily armed police raided an MDC house in Harare, claiming that they were searching for weapons. They forced the caretaker to dig up the garden with his hands but did not find anything.

Two days later the chairman and chief executive of the National Association of Non-Governmental Organisations, an umbrella body of the country’s charities, were arrested at Victoria Falls airport. They were accused of holding an illegal political meeting and held for two days before being released on bail.

On Tuesday there were two abduction attempts on other MDC officials. One escaped when she screamed for help but the transport manager, Pascal Gwenzere, was dragged out of his township home by armed men in civilian clothes. He is still missing.

A day later two officials at the Zimbabwe Election Support Network were arrested in a village in the remote northwest of the country where they were holding an election education workshop. They were charged with holding an illegal meeting.

Teachers in rural areas have been reporting the redeployment of Zanu (PF) youth militias in schools. The incidents, which were once rife, had become sporadic since the unity Government was formed.

Recent events have raised fears that the lull in violence will be short-lived. “What we are seeing now was exactly how the violence began in the campaign for the presidential run-off elections between March and June last year,” Nelson Chamisa, a spokesman for the MDC, said. “We are beginning to see the formation of another storm of violence.”

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