When the call of nature comes, it cannot always be denied. Few have answered it in such an unfortunate fashion as Alois Mabhunu.

While on duty at a trade fair, the Zimbabwean police sergeant could not hold on and allegedly dashed to the nearest toilet – disastrously, it transpired, as it was reserved for President Robert Mugabe.

He was arrested and imprisoned on suspicion of invading the presidential privy.

The incident happened at the annual Zimbabwe International Trade Fair (ZITF) in the western city of Bulawayo, according to local radio station VOP.

Under the headline "Never Use Toilet Reserved For President!" VOP's website reported that Mabhunu was on duty during the fair's official opening by Mugabe and Jean-Louis Ekra, the president of Afreximbank.

"Mabhunu, due to the call of nature, rushed to the toilets reserved for Mugabe and his guest Ekra, but was stopped by other officers guarding the toilets," VOP said.

"Under intense pressure from the call of nature, the officer forced his way in and managed to relieve himself. He was arrested on 7 May after a report was made to Mugabe's security men and to senior police officers in the city."

Mabhunu, a murder detective, is in police detention at barracks on the outskirts of Bulawayo. The legality of the case against him was challenged by Beatrice Mtetwa, a leading Zimbabwean human rights lawyer.

"There has to be a law saying the toilet is the president's, but this was a public one," she said. "They will have had to issue a proclamation in the government gazette specifying it. I bet they didn't do that."

Mugabe's personal space – and reputation as father of the independent nation – are fiercely protected. Several motorists are said to have been assaulted by his security personnel for not giving way to the presidential motorcade.

In March, Douglas Mwonzora, a leading member of the Movement for Democratic Change, was waiting for a court session to start when he looked at a portrait of Mugabe and said: "How are you, father? How is your health?"

This was greeted with mirth in the courtroom but he was charged with insulting the president, an offence punishable by up to a year in prison.

Another man will go on trial next month charged with posting an offensive message on Facebook. Vikas Mavhudzi allegedly wrote on prime minister Morgan Tsvangirai's 'wall': "What happened in Egypt is sending shockwaves to all dictators around the world. No weapon but unity of purpose. Worth emulating, hey."