Zimbabwe's opposition has called on South African President Thabo Mbeki to stand down as a mediator in the wake of the elections crisis.
MDC leader Morgan Tsvangirai told the BBC Mr Mbeki should be replaced by Zambian President Levy Mwanawasa.
Mr Tsvangirai also hinted he could put President Robert Mugabe on trial for what he described as the suppression of people who voted against him.
Zimbabwe's government on Thursday accused Mr Tsvangirai of treason.
The MDC leader also said the party came close to an agreement with President Mugabe's Zanu-PF that would have removed him from power.
Mr Tsvnagirai told the BBC the MDC was approached by the president's envoys a day after the disputed election calling for a government of national unity but the talks came to a close after three or four days.
He said talks failed when it became clear there were "others in the establishment who did not want to accept that".
'Orgy of violence'
Mr Tsvangirai says he won the recent presidential election outright. The results have not been published.
He told a news conference in Johannesburg: "We want to thank President Mbeki for all of his efforts but President Mbeki needs to be relieved of his duties."
Hundreds of opposition supporters have been displaced in Zimbabwe
Mr Tsvangirai said he had called on the regional Southern African Development Community, under the chairmanship of Mr Mwanawasa, to lead a new mediation effort.
He said Zimbabwean President Robert Mugabe had "unleashed an orgy of violence against the people".
Mr Tsvangirai suggested a UN crimes court similar to those in Sierra Leone and the Democratic Republic of Congo.
"As I speak, our people are being murdered, homes burned, children molested, women raped," he said.
Mr Tsvangirai has previously said there should be no "witch hunt" against Mr Mugabe.
But in an interview later with the BBC's Allan Little, Mr Tsvangirai said he had to revisit the policy as he was "so disappointed with recent events... with the current campaign to repress the people".
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.
President Mbeki had defended his record on Zimbabwe at the UN in New York on Wednesday.
Mr Mbeki met President Mugabe last Saturday and afterwards said there was "no crisis" in Zimbabwe. He defended those remarks 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
"The solution to the problem of Zimbabwe lies in the hands of the people of Zimbabwe," he said.
Earlier on Thursday, South Africa urged Zimbabwe's electoral commission to release the results of last month's presidential election.
The call was echoed by the G8 group of major industrialised countries, the European Union and US Secretary of State Condoleezza Rice, who said Zimbabwe had become an "abomination".
"It's time for Africa to step up," Ms Rice said. "Where is the concern from the African Union and from Zimbabwe's neighbours about what is going on in Zimbabwe?"
The Zimbabwe Electoral Commission says it cannot release the results until it investigates anomalies - a partial recount takes place this weekend.
Zimbabwe's government has meanwhile stepped up its campaign against Mr Tsvangirai.
Thabo Mbeki has defended his role on Zimbabwe
Justice Minister Patrick Chinamasa alleged he was 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 also said it had details of a letter from UK Prime Minister Gordon Brown to Mr Tsvangirai assuring him that Britain had lobbied southern African leaders to hold an urgent summit on Zimbabwe and that London would impose more sanctions.
"It is clear from the correspondence that Tsvangirai along with Brown are seeking regime change in Zimbabwe, and on the part of Tsvangirai. 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.
He told Allan Little that Mr Chinamasa was an "injustice minister" as he had lost his seat.
"There is no treason I have committed - I have not colluded with anyone," he said.
Meanwhile, South African officials have confirmed to the BBC that a Chinese ship anchored off the port of Durban does contain arms destined for Zimbabwe.
The officials said South Africa could not interfere in a trade deal between two nations but only ensure proper procedures were followed.
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