FACTBOX: Possible scenarios in Zimbabwe's elections

HARARE (Reuters) - Zimbabwe's President Robert Mugabe faces the biggest challenge of his 28 years in power in presidential, parliamentary and local elections on Saturday.

With the once prosperous country's economy in ruins, the 84-year-old leader faces fierce competition from his former finance minister, Simba Makoni, and opposition MDC leader Morgan Tsvangirai. Here are some possible scenarios for the outcome of the vote.

MUGABE WINS, OPPOSITION DISPUTES RESULT

Mugabe wins the presidential poll outright by taking more than 51 percent, avoiding a second round runoff. Seen by analysts as the most likely outcome, this would be based on support from his ruling ZANU-PF party's traditional rural strongholds. But most analysts believe Mugabe would also use his power to manipulate the vote.

Tsvangirai's MDC is likely to reject such a result as rigged and some of its supporters could take to the streets. However a Kenyan scenario of prolonged protests and bloodshed seems unlikely.

The government would deploy riot police to ruthlessly suppress the demonstrations and could threaten sweeping arrests and a state of emergency. Most analysts would expect demonstrations to quickly subside. Mugabe may launch a purge of Makoni supporters in the party and government.

MUGABE IS FORCED INTO A RUN-OFF

Mugabe claims the highest number of votes but fails to reach 51 percent, forcing a second round run-off, which would most likely be against Tsvangirai. The Makoni campaign has said it would rally behind the single opposition candidate if this happened.

If there is a run-off, Mugabe would be expected to deploy ZANU-PF militants and independence war veterans across the country to swing the vote for their leader, raising the prospect of violent clashes with defiant MDC supporters in the three-week hiatus between votes.

A run-off is likely to end with Mugabe being declared victor, leaving political tension and no prospect of saving the collapsing economy. Western powers would likely tighten sanctions against Mugabe's government and his power within the party may be weakened.

TSVANGIRAI, MAKONI BOTH CLAIM VICTORY

Both Tsvangirai and Makoni claim they have won the elections and their supporters launch street protests to force Mugabe to concede defeat.

The government accuses the opposition leaders of treason and of trying to subvert the constitutional order but the opposition forms an alliance to press for a settlement, giving new impetus to street protests and sparking wide international condemnation of Mugabe.

MAKONI WINS

If Makoni's reported but undeclared support in the ruling party and opposition ranks turns into votes, and the army and security organs accept him as a unifying figure, Mugabe may accept defeat.

TSVANGIRAI WINS