Johannesburg, South Africa — Talks between the President Mugabe of Zimbabwe and the opposition Movement for Democratic Change were facing collapse last night.

Representatives of Mr. Mugabe’s Zanu-PF party disclosed that they were mandated only to offer the opposition leader, Morgan Tsvangirai, a vice-presidency, with no executive power.

The offer came despite Mr. Tsvangirai decisively beating Mr. Mugabe in the first round of the presidential poll in March, and Zanu-PF losing its parliamentary majority. The MDC pulled out of the presidential run-off last month in the face of a campaign of violence against its supporters, more than 100 of whom were killed, some in appalling brutal ways.

The MDC insists it is the rightful winner of the elections and is adamant that it will only accept a transitional authority, with Mr. Tsvangirai in control, leading to fresh elections in about two years.

“I guess we knew Zanu-PF was not serious and we have never expected anything from [South African President Thabo] Mbeki,” an MDC source in Harare said.

Mr. Mbeki, who is mediating the negotiations, wanted to see a government of national unity emerge from the process.

Two officials said that the Zanu-PF negotiators, the justice minister, Patrick Chinamasa, and the social welfare minister, Nicholas Goche, were flying back to Harare last night from Pretoria, where the negotiations were being held, while MDC officials said Mr. Tsvangirai had flown to South Africa from Zimbabwe.

Perhaps the only surprise is that the talks, which were scheduled to last for two weeks, got into difficulties so soon after they started last Thursday.

The agreement last week on a memorandum of understanding about the process triggered a wave of optimism in some quarters.