U.S. Ambassador James McGee, among those detained on Thursday, accused the government on Friday of using food aid to try to win votes.
"We are dealing with a desperate regime here which will do anything to stay in power," he told a video conference call from Harare. Washington has blamed the diplomats' detention firmly on Mugabe's government.
The United States and former colonial power Britain say Zimbabwe is trying to intimidate Tsvangirai's supporters. France's foreign ministry also said on Friday it was "extremely worried by the climate of intimidation and violence."
The opposition says 65 people have been killed in violence since the first round of voting. Mugabe blames his opponents.
Mugabe's government suspended the work of all international aid agencies in the southern African nation on Thursday, saying some of them were campaigning for the opposition.
Britain and the European Union demanded the lifting of the ban. U.N. officials said it would increase suffering and CARE, one of the agencies whose work has been suspended, said millions of aid-dependent Zimbabweans were at risk.
EU aid commissioner Louis Michel said: "I am deeply distressed to think that hundreds of thousands of people who depend on aid from the European Commission and others for their very survival now face an even more uncertain future."
Zimbabwe, once one of Africa's most prosperous countries, has seen food production plummet since 2000 when Mugabe's government began seizing thousands of white-owned farms as part of a land redistribution programme to help poor blacks.
Many of the farms have ended up in the hands of Mugabe loyalists, and the country now faces chronic food shortages. It has had to rely on handouts and imports to feed its people.
Mugabe blames sanctions imposed by Western countries for the collapse of the once prosperous economy. The opposition says he ruined Zimbabwe through mismanagement.
The Southern African Development Community, a regional grouping of 14 nations, including Zimbabwe, is sending observers to monitor the run-off.
