FACTBOX: What next in Zimbabwe's political crisis?

(Reuters) - Below are answers to some questions on Zimbabwe's political crisis ahead of a presidential run-off from which opposition leader Morgan Tsvangirai has withdrawn, saying violence by President Robert Mugabe's supporters meant it could not be fair.

WHAT HAPPENS NOW?

Zimbabwe's government says the run-off will take place on Friday without Tsvangirai. Mugabe is virtually guaranteed to win the contest and the 84-year-old Zimbabwean leader would then be sworn in for another five-year term.

Tsvangirai beat Mugabe in the first round of voting, but according to official figures fell short of the outright majority needed to avoid a run-off.

WHAT ARE THE CHANCES OF A POSTPONEMENT

Mugabe's government is adamant that the vote will take place and preparations are under way for voting to start at 1 a.m. EDT on Friday. Polls will close at 1 p.m. EDT.

Regional grouping SADC (Southern African Development Community) has called for the election to be postponed, as have many African leaders and Western countries. Impatience with Mugabe is growing on the continent and African icon Nelson Mandela also weighed in this week, criticizing "the tragic failure of leadership in our neighboring Zimbabwe".

WHAT HAPPENS IF THE ELECTION GOES AHEAD?

Zimbabwe's government says the winner of the election, almost certainly Mugabe, will be sworn in soon.

Tsvangirai has said if Mugabe goes ahead with the election and declared himself president, he would be shunned as an illegitimate leader who killed his own people.

Governing the country may be tricky for Mugabe because the opposition won control of parliament in the March 29 general election. Mugabe's ruling party has challenged some of those results.

WHAT IS THE IMPACT ON ZIMBABWE AND SOUTHERN AFRICA?

The prospects of reversing Zimbabwe's economic meltdown are slim without a change in government. Mugabe refuses to consider reforms and Western powers are unlikely to provide the billions of dollars in development aid needed to bail out the economy.

Zimbabwe's neighbors could be swamped with an even bigger influx of refugees.

An estimated three million Zimbabweans are in South Africa, where there are rising anti-immigrant feelings. More than 60 African migrants were killed in recent attacks by mobs there.

Investors keen to invest in Zimbabwe will keep plans on hold despite a widespread belief that the once-prosperous economy could bounce back quickly under a new government.

HOW WILL AFRICA AND THE WORLD REACT?

The 14-nation SADC is under increasing pressure to help settle the crisis. The African Union will discuss Zimbabwe at a summit in Egypt at the weekend.

Britain is pushing for Mugabe's government to be declared illegitimate and wants broader sanctions against the Zimbabwean ruler and his top officials. It says it is already preparing its own.

Military intervention by SADC or the African Union is not seen as an option. Any such move is likely to face strong opposition from South Africa, the regional political and economic powerhouse.

WILL THERE BE MORE VIOLENCE?

Tsvangirai has sought refuge at the Dutch embassy and police raided his party's headquarters in Harare. He has said he expects voters to be forced out to polling stations by Mugabe's supporters on Friday.

The MDC and trade unions could take to the streets to protest against Mugabe's government, but that would probably prompt a heavy backlash by well-equipped security forces. Tsvangirai and other MDC officials were beaten in an aborted March, 2007 rally.

COULD THERE STILL BE NEGOTIATIONS?

Although Tsvangirai has said he would not negotiate with Mugabe if the vote goes ahead, he could still be under pressure to do so from some quarters.

South African President Thabo Mbeki has urged the two sides to hold talks. South African media had reported Mbeki, who has mediated the Zimbabwe crisis for more than a year, favored the formation of a unity government. The idea has not taken off.

HOW LONG WILL MUGABE STAY IN POWER?

Mugabe has said he wants to stay on until he is sure that it is impossible to reverse a land redistribution program that saw thousands of white-owned farms seized and given to poor blacks -- one of the policies blamed for Zimbabwe's collapse.

Some believe that victory in the election could allow him to bow out sooner rather than later, by departing from a position of strength in favor of a hand-picked successor, and with the opposition in disarray.

(Reporting by Harare and Johannesburg bureaux; Editing by Matthew Tostevin)