MDC leader Morgan Tsvangirai has been detained by police during campaigning for the country’s presidential election, his party says.
The Movement for Democratic Change leader was stopped at a roadblock north of Bulawayo at 1000 GMT, party spokesman Nelson Chamisa said.
Armed police surrounded an MDC convoy and refused to let them leave their vehicles, said Mr Chamisa.
“It appears they want to disrupt our campaign programme,” he said.
It comes as Zimbabwe prepares for a run-off election between Mr Tsvangirai and President Robert Mugabe on 27 June.
Aid operations suspended
Mr Chamisa called for Mr Tsvangirai’s immediate release, adding that without the intervention of the United Nations and the international community it would be impossible to hold free and fair elections in Zimbabwe.
The Zimbabwean government has also come under criticism for restricting the work of aid agencies operating in the country.
The operations of one of the largest groups – Care International – have been suspended amid allegations the group had been campaigning for the opposition.
Care denies that it “has encouraged or tolerated any political activity”.
Mr Mugabe is currently attending a UN-sponsored summit on the global food crisis in Rome, where he blamed Western sanctions for Zimbabwe’s economic woes.