(Adds details about IGAD meeting)

By Duncan Miriri and Helen Nyambura-Mwaura

NAIROBI, Feb 5 (Reuters) - Kenya's opposition on Tuesday threatened new street protests if a meeting of regional leaders chaired by the government goes ahead this week while the two sides are locked in political negotiations.

The rivals on Tuesday began the toughest part of their talks so far to try to end the crisis over a disputed election that has killed at least 1,000 people and hurt the east African country's reputation for stability and economic promise.

Former U.N. chief Kofi Annan, mediating talks, said the opposition threat should not have been made in light of the talks and a pledge by both sides to avoid antagonistic proclamations.

"We have a demand that the parties avoid provocative statements outside negotiations," Annan told reporters. "We are going to be vigilant on that. I think there is a clear understanding that it should not have been done and there will be no mass protests."

The opposition had attacked plans to hold a meeting in Nairobi of the seven-member east African regional bloc IGAD, the rotating chairmanship of which is held by Kenya.

Foreign ministers are due in Kenya on Wednesday, with talks due the next day. IGAD last week said it supports Annan's mediation, and offered any help needed.

It was not immediately clear whether IGAD's heads of state would come later -- as is normally the case -- to endorse the statements agreed on by their foreign ministers. An IGAD official who declined to be named said it was likely.

"If the IGAD meeting goes on in spite of our call for it not to go on, we shall call upon Kenyans to come out in their big numbers for a peaceful demonstration in Nairobi to strongly protest," ODM secretary-general Anyang' Nyong'o said.

The government has banned street protests, and earlier ones have led to looting, rioting and a bloody police crackdown.

SOUTH AFRICA

South Africa on Tuesday strongly attacked President Mwai Kibaki's government for raising objections to a prominent anti-apartheid figure as a mediator in Kenya's crisis.

Cyril Ramaphosa, a Johannesburg business tycoon, was nominated by Annan to lead longer-term negotiations in Kenya. But he pulled out on Monday after the government accused him of having business links with opposition leader Raila Odinga, allegations he denied.

"The reasons given by the (Kenyan) government are rejected with the contempt they deserve," Deputy Foreign Minister Aziz Pahad told reporters.

What started as a dispute over the Dec. 27 re-election of Kibaki has laid bare divisions over land, wealth and power dating from colonial rule then stoked by Kenyan politicians.

Most of the deaths, in one of Kenya's darkest moments since independence from Britain 44 years ago, have come from cycles of ethnic killings, adding to fatalities from police clashes with protesters.

Odinga insisted on external mediation, which led to the Annan mission that so far has produced commitments to end violence and help those displaced.

On Tuesday, Annan pushed the two sides to focus on the third item on their agenda -- "the political crisis arising from the disputed presidential electoral results."

Odinga argues the president was illegally returned to office through vote-rigging, and Nyong'o said the IGAD regional meeting would "legitimise Kibaki's position through the back door."

International observers have said the vote counting was so chaotic that it was impossible to tell who won.

The government says Kibaki was elected under Kenya's laws.

On Tuesday, Kenya's Red Cross said the death toll had risen to more than 1,000, while some 304,000 Kenyans have been displaced by the crisis -- and the figure was expected to rise.

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