May 16 (Reuters) - Zimbabwe said on Friday it would hold a delayed presidential election run-off on June 27 in which the opposition hopes to oust veteran leader Robert Mugabe after almost 30 years in power.

The long-awaited second round follows a disputed March 29 election in which official results showed opposition Movement for Democratic Change (MDC) leader Morgan Tsvangirai beat Mugabe, but not by enough votes to avoid a run-off.

WHAT WILL HAPPEN IN THE CAMPAIGN?

After weeks of uncertainty, Tsvangirai, who has been abroad since April 8 seeking to keep international pressure on Mugabe, has said he will take part in the run-off and will return to Zimbabwe on Saturday.

But police have banned a rally Tsvangirai was scheduled to address in Zimbabwe's second city of Bulawayo on Sunday. The opposition party has said it will challenge the ban in court.

Mugabe has said he would not allow what he says is a Western-backed opposition party to come to power.

HOW FAIR WILL THE VOTE BE?

Tsvangirai's MDC, civic groups and regional bloc SADC have said post-election violence in Zimbabwe has created conditions making it difficult for a free and fair run-off vote.

The opposition party says 40 of its members have been killed in political violence across the country, while thousands of homes have been destroyed. Mugabe and his ruling ZANU-PF party blame violence on the opposition.

ZANU-PF officials and security chiefs have been among those calling for a stop to violence, but it is unclear whether that will have much effect ahead of the vote.

Civic groups say political violence will lead to another disputed election and the opposition is worried it might not be able to campaign freely ahead of the run-off.

There are signs the opposition might not enjoy the unprecedented access to state media it had before the March 29 poll. Mugabe's government fired the head of the state broadcasting monopoly on Wednesday, amid reports that he had granted the opposition too much access on state television and radio.

WILL WESTERN POLL OBSERVERS BE ALLOWED IN?

The Zimbabwe government has maintained its position that no Western observers would be invited for the poll.

Tsvangirai has backed down on his earlier demand for participating in the run-off that more international election observers, including the U.N, be allowed to monitor the election.

Regional group SADC has said it could send more observers for the run-off than it did for the first round.

WHO WOULD WIN A RUN-OFF?

On the face of it, Tsvangirai would appear to be in a strong position to win a fair election. However, the opposition, human rights groups and Western countries accuse Mugabe of launching a campaign of violence and intimidation to try to secure victory in a second round.

WHAT IF MUGABE STAYS IN POWER?

Zimbabwe's economic crisis is likely to worsen and analysts say there will be few prospects of serious political change.

Western sanctions have failed to weaken Mugabe and he is likely to tighten his grip on power by purging opponents within and outside his party.

Official results show ZANU-PF lost control of parliament to the opposition for the first time in the March 29 parallel vote, which would make governing more difficult for Mugabe. A minority ZANU-PF cabinet would face problems passing legislation, which would need cooperation from the opposition.

The opposition does not have the two-thirds majority needed to impeach the president or change the constitution.

WHAT IF TSVANGIRAI WINS?

Analysts say the end of Mugabe's rule would probably bring badly needed international aid. The U.S. ambassador to Zimbabwe has said Zimbabweans should expect a package worth billions of dollars if a democratic government that embraces free markets is formed.

Tsvangirai has spoken in broad terms about creating conditions for foreign investment of $10 billion. He has not spelled out how he would ease Zimbabwe's economic crisis marked by severe shortages of basic goods and the world's highest inflation rate of 165,000 percent.

WHAT IF THERE IS PROLONGED POLITICAL DEADLOCK?

Zimbabwe could slip at least briefly into the kind of violence that affected Kenya after the disputed December election, although Mugabe's security forces are likely to quickly end unrest.

Talk has been floated of a possible national unity government as a way out of the crisis, but each side would insist on leading such an administration. (Reporting by Harare bureau;)

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