Zimbabweans will go to the polls to decide the second round of a hard-fought presidential contest later this month after opposition leader Morgan Tsvangirai beat incumbent Robert Mugabe in the first round in March.

But opposition campaigners, human rights groups and Western powers complain of a brutal harassment campaign with Tsvangirai detained four times in the past week. Tsvangirai says 66 of his followers have been murdered.

In Friday's full-page advertisement in the Financial Times and South African daily Business Day, academics, former prime ministers and religious leaders said they were "deeply troubled" by the reports of intimidation, harassment and violence.

"As Africans we consider the forthcoming elections to be critical. We are aware of the attention of the world," the appeal, signed by some of Africa's most well-known figures, including Nobel laureate Desmond Tutu and Senegalese musician Youssou N'Dour, said.

"We call for an end to the violence and intimidation, and restoration of full access for humanitarian and aid agencies."

They also called for an "adequate number of independent electoral observers" both during and after the June poll.

British Prime Minister Gordon Brown called separately on Friday for Zimbabwe to allow a return of aid workers, U.N. agencies and non-governmental organisations, along with more international election observers.

Friday's appeal was supported by the Mo Ibrahim Foundation, founded by the telecom entrepreneur to support governance in Africa.