(Adds South African union comment, paragraphs 16-17)

By Stella Mapenzauswa

JOHANNESBURG, July 23 (Reuters) - Zimbabwe's opposition MDC and President Robert Mugabe's ruling ZANU-PF will begin negotiations on a power-sharing deal in earnest on Thursday, officials said on Wednesday.

The rivals opened preliminary talks on Tuesday, South African President Thabo Mbeki's spokesman said, but he declined to say what was discussed.

"I won't talk about what it is. I will say talks resumed, and talks are continuing," Mukoni Ratshitanga said. "They have started more or less. (They will be) in earnest tomorrow."

Mbeki, mediating in the crisis, secured a framework deal between Mugabe and opposition leader Morgan Tsvangirai on Monday for talks to end the deadlock since Mugabe's re-election on June 27 in a poll boycotted by the opposition because of violence.

Pressure for power-sharing has come from regional states concerned by the political and economic crisis that has forced millions of refugees to flee to Zimbabwe's neighbours.

Zimbabwe's state-run Herald newspaper said Justice Minister Patrick Chinamasa and Public Services Minister Nicholas Goche would represent ZANU-PF at negotiations, while MDC Secretary General Tendai Biti and Deputy Treasurer Elton Mangoma would attend for Tsvangirai's party.

A splinter faction of the MDC would also have two negotiators at the talks.

The MDC says 120 of its supporters have been killed since a first round of elections on March 29, in which Tsvangirai beat Mugabe but without the absolute majority to avoid a run-off. Mugabe blames the opposition for the bloodshed.

DIFFERENCES

The main aim of the Pretoria talks will be the creation of a government of national unity, but the two sides differ on who should lead it and how long it should stay in power.

Mbeki has said the Zimbabwean parties face a tight two-week deadline to conclude the talks, which are expected to be tense and possibly acrimonious. The MDC has accused Mugabe of violating human rights and rigging elections.

The European Union on Tuesday agreed additional sanctions on Zimbabwe to target 37 more individuals and four companies linked to the government.

Britain said on Wednesday it would press on with sanctions.

"We'll continue with the policy of sanctions to continue the pressure that is necessary so there is a fair outcome of any negotiations to the benefit of the people of Zimbabwe," Prime Minister Gordon Brown said at a joint news conference with Kenyan Prime Minister Raila Odinga at Downing Street.

Odinga, whose own country's election last year was disputed and violent, said Mugabe should be given a decent exit.

"A decent exit would mean that Mugabe is given a role to play for an interim period of time -- I understand his friends are suggesting about 24 months -- after which elections will be held in Zimbabwe. But in that period of time he can stay on as a ceremonial president if you like with an executive prime minister until the elections are held," Odinga said.

The powerful Congress of South African Trade Unions vowed on Wednesday to protest against Mugabe if he attended a SADC summit in South Africa next month.

"When Mugabe comes to South Africa, we must tell him that he's not a president of his country. He has lost elections. As workers we are not going to service him," COSATU General Secretary Zwelinzima Vavi said at a rally in Johannesburg.

Mugabe, 84, says his critics are puppets of the West.

Zimbabwe's economy has been in freefall since 2000, with the world's highest inflation at over two million percent. (Additional reporting by Nelson Banya in Harare, Phakamisa Ndzamela in Johannesburg and Katherine Baldwin in London; Writing by Marius Bosch; Editing by Paul Simao and Giles Elgood)

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