Zimbabwe's main opposition leader on Monday called on President Robert Mugabe to step down after 28 years in power following an historic victory by the opposition in parliament.

The leader of the Movement for Democratic Change (MDC), Morgan Tsvangirai, in South Africa as the authorities in Harare kept more than 200 opposition supporters behind bars in defiance of a court order.

"Mugabe should concede that he cannot be president without controlling the parliament," Tsvangirai told reporters in Johannesburg.

"The old man must go and have an honourable exit," he added.

Tsvangirai has proclaimed himself the winner against Mugabe in a presidential election on March 29 from which official results have never been released despite mounting international pressure on Zimbabwe to do so.

UN to discuss Zimbabwe

The UN Security Council was set to discuss the Zimbabwe crisis on Tuesday, with permanent members Britain and the United States putting pressure on Mugabe to step down and for the United Nations to intervene.

The main US envoy for Africa, Assistant Secretary of State Jendayi Frazer, has threatened Zimbabwe with UN sanctions if the crisis continues and Britain has called for a UN mission to verify human rights abuses.

A spokesperson for the Zimbabwe Electoral Commission, Utloile Silaigwana, said a partial recount of ballots from the elections would be completed by Monday but could not say when exactly the results would be released.

"I can't put a timeframe on it," Silaigwana said.

The opposition accuses Mugabe of ordering the recount in order to buy time to unleash a campaign of violence in rural areas that voted against him so as to secure victory in a possible second round of the presidential election.

The opposition says that at least 15 of its supporters have been killed in politically motivated attacks since the elections, hundreds injured and thousands more forced to flee their homes by pro-government militias.

Inflation tops 165 000 percent

The government has dismissed these reports as "lies" aimed at stirring up unrest in Zimbabwe and has accused Western countries of seeking to undermine Zimbabwe's independence and ruining the country with their sanctions.

The political crisis in this troubled southern African state comes on top of an economic collapse, with shortages of most basic foodstuffs and an inflation rate officially put at 165 000 percent — the highest in the world.

Official results from parliamentary elections also held on March 29 show that Mugabe's ruling Zanu-PF party has lost control of the legislature for the first time since independence from Britain in 1980.

Tsvangirai said on Monday that the MDC would work together with a breakaway faction of the movement to form a majority in parliament. Zimbabwe has a strong presidential system of government but analysts say the president still needs the support of parliament to pass key legislation.

The opposition leader also called for the United Nations to send a special envoy to investigate the violence and ease a transition of power, accusing the government of "crimes against humanity."

The fate of 215 opposition activists rounded up by armed riot police in a swoop on the headquarters of the MDC on Friday remained uncertain despite an international outcry calling for their release.

"We haven't had any access to them. We served the court order on the various police stations but they are still being held and this is in contempt of the court order," Alec Muchadehama, a lawyer for the detainees, told AFP on Monday.

Muchadehama said some of the detainees needed urgent medical attention. Also on Monday, a group of veterans from the country's national liberation war that has been critical of Mugabe in the past accused the 84-year-old leader of imposing "a fascist dictatorship."

In a statement issued on Sunday, Louise Arbour, the UN High Commissioner for Human Rights, condemned politically motivated attacks and called on both government and opposition supporters to refrain from violence.

Meanwhile, the two factions of the Movement for Democratic Change (MDC) have agreed to work together in parliament, said the MDC's Morgan Tsvangirai on Monday. He was speaking at a media briefing at Lanseria airport near Johannesburg.

Tsvangirai said the formation was significant because the party was now in control in Zimbabwe.

"Zanu-PF has the minority seats in parliament now, so it is now the opposition." he said. Tsvangirai said President Robert Mugabe had to accept that the majority ruled.

"To be President you have to be in control of parliament. He lost that so he can no longer be president of Zimbabwe," he said.

Arthur Mutambara, head of a faction of the MDC, also addressed the media. He said the two faction's decision to work together was in the national interest.