Zimbabwe's main opposition party has lost a legal bid for its secretary general to be freed from custody.The Movement for Democratic Change argued that Tendai Biti - who is expected to face a treason charge - was not being legally detained.But a high court judge said Mr Biti's detention on his return from South Africa was lawful.The MDC's Morgan Tsvangirai faces incumbent Robert Mugabe in a presidential run-off vote on June 27.Mr Tsvangirai won more votes than Mr Mugabe in the first round on 29 March but not the 50% needed for outright victory.International concernProsecutors said they were ready to bring Mr Biti, who was arrested in Harare last week, to court on Wednesday. If convicted of treason he faces the death penalty or life in prison.Tuesday's hearing came as a senior UN official met Mr Mugabe in Zimbabwe to discuss the country's political and humanitarian situation.Haile Menkerios, UN assistant secretary general for political affairs responsible for Africa, is expected to meet other politicians during his five-day visit.The MDC says Mr Biti's continued detention is intended to frustrate the party's campaign before next week's run-off.Authorities have said they plan to charge Mr Biti for allegedly writing a document containing details of a plot to rig the March election.They also accuse him of "communicating false information prejudicial to the state" for proclaiming an MDC victory in Zimbabwe's first round vote ahead of official results.Mr Tsvangirai has been briefly detained a number of times over the last few weeks on the campaign trail.There has been growing international concern that political violence will make a free and fair vote impossible.The opposition, human-rights groups and some Western governments accuse Mugabe supporters of directing a campaign of intimidation against the MDC.Mr Mugabe, 84, has said the MDC will never gain power in his lifetime.The opposition says at least 66 of its supporters have been killed and some 25,000 forced to flee their homes.But Zimbabwean Justice Minister Patrick Chinamasa has accused the opposition of leading a violent campaign.Source: BBC