Zimbabwe's opposition leader says his party came close to an agreement with the ruling Zanu-PF to remove President Robert Mugabe from power.
Morgan Tsvangirai said his MDC party was approached by presidential envoys about a possible unity government the day after the disputed election.
The talks broke down after a few days, Mr Tsvangirai told the BBC.
On Friday President Mugabe is expected to make his first major speech since last month's elections.
He will speak at celebrations in the capital Harare to mark 28 years since independence from Britain and the end of white minority rule.
Meanwhile, South Africa has said it will not stop a shipment of weapons from China being transported across its territory to Zimbabwe.
Reports say a Chinese cargo ship anchored off the South African port of Durban is carrying 3m rounds of ammunition and 1,500 rockets.
Unity government talks
In an interview with the BBC's Alan Little, Mr Tsvangirai said the MDC had been "absolutely" prepared to give Zanu-PF officials, including Mr Mugabe, guarantees that they would not be prosecuted by the proposed government of national unity.
Hundreds of opposition supporters have been displaced in Zimbabwe
"We were prepared to consider the issue of an inclusive government including some members of Zanu PF," he said.
"In fact they were suggesting how many and they were talking about a panel from which we were going to choose."
The unity government talks failed when it became clear there were "others in the establishment who did not want to accept that".
Mr Tsvangirai remains adamant he won the 29 March presidential election outright.
But the Zimbabwe Electoral Commission says it cannot release the results until it investigates anomalies - a partial recount takes place this weekend.
'Orgy of violence'
Earlier, the MDC leader had told a news conference in Johannesburg that South African President Thabo Mbeki "needs to be relieved of his duties" as a mediator in the crisis.
Mr Mbeki should be replaced by Zambian President Levy Mwanawasa, Mr Tsvangirai said.
Mr Mugabe had "unleashed an orgy of violence against the people", Mr Tsvangirai said.
"As I speak, our people are being murdered, homes burned, children molested, women raped," he said.
Mr Tsvangirai also said he planned to return to Zimbabwe from his current base in Botswana but would not give a timeframe.
"I am not in exile. I am going back to Zimbabwe. I have not run away from the people. I am with them in the struggle," he said.
Despite increasing concerns of violence in Zimbabwe, South African officials have said they can do nothing to prevent a Chinese shipment of arms from being delivered to the land-locked country.
The An Yue Jiang is reportedly laden with ammunition, mortars and rockets.
The An Yue Jiang, which is anchored just outside Durban harbour, is reportedly carrying nearly three million rounds of ammunition, about 3,500 mortars, and 1,500 rockets destined for Harare.
The South African newspaper Beeld said it had a copy of the ship's cargo inventory which was finalised three day's after Zimbabwe's disputed election.
Mr Tsvangirai described the import of arms rather than food for needy Zimbabweans as "disgusting".
"It only shows the warped nature of the priorities of this regime: that they are more preoccupied with the defence-power project than anything else," he said.
South African officials have told the BBC they cannot interfere in a trade deal between two nations but only ensure proper procedures were followed.
President Mbeki has defended his "quiet diplomacy" policy on Zimbabwe at the UN in New York, saying dialogue was essential.
It is clear from the correspondence that Tsvangirai along with Brown are seeking regime change in Zimbabwe
Patrick Chinamasa, Zimbabwe Justice Minister
On Thursday, South Africa, the G8 group of major industrialised countries, the European Union and the US urged Zimbabwe's electoral commission to release the presidential election results.
US Secretary of State Condoleezza Rice said Zimbabwe had become an "abomination".
"It's time for Africa to step up," she said. "Where is the concern from the African Union and from Zimbabwe's neighbours about what is going on in Zimbabwe?"
Zimbabwe's government has meanwhile stepped up its campaign against Mr Tsvangirai, with Justice Minister Patrick Chinamasa accusing him of working with Britain to bring about "regime change".
The state-controlled Herald newspaper accused Mr Tsvangirai of approaching the UK government to discuss possible military intervention.
The Herald said it had details of a letter from UK Prime Minister Gordon Brown to Mr Tsvangirai assuring him London would impose more sanctions on Zimbabwe.
"It is clear from the correspondence that Tsvangirai along with Brown are seeking regime change in Zimbabwe... This is treasonous," Mr Chinamasa is quoted as saying.
The UK embassy in Harare said the correspondence was "a forgery". Mr Tsvangirai also rejected the treason allegations, describing Mr Chinamasa as an "injustice minister" because he had lost his seat.
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