By Cris Chinaka

HARARE (Reuters) - Heavily armed riot police deployed in potential election flashpoints in Zimbabwe on Tuesday on the eve of a poll showdown between President Robert Mugabe and Prime Minister Morgan Tsvangirai that remains too close to call.

State radio said thousands of officers had been sent to the central Midlands province, while trucks of police carrying automatic rifles and grenade launchers patrolled in the restive Harare townships of Highfield and Mbare.

The run-down districts of the capital are hotbeds of support for Tsvangirai and were at the center of several weeks of post-election violence in 2008, in which 200 people linked to his Movement for Democratic Change (MDC) were killed.

This year's presidential and parliamentary race brings the curtain down on four years of fractious unity government. It has been marked by allegations of threats and intimidation by security forces but there have been no reports of violence.

With no reliable opinion polls, it is hard to tell whether 61-year-old Tsvangirai will succeed in his third attempt to unseat his 89-year-old rival, who has run the southern African nation since independence from Britain in 1980.

Both the MDC and Mugabe's ZANU-PF party predict landslide victories. However, it is possible neither leading candidate will emerge an outright winner, triggering a September 11 run-off. That is a nightmare scenario for many of Zimbabwe's 13 million people who remember the 2008 violence.

Western election observers have been barred, leaving the task of independent oversight to 500 regional and 7,000 domestic monitors. The final results must be released within five days but may come sooner.

In an editorial in the domestic News Day newspaper and the Washington Post, Tsvangirai urged African monitors not to give the vote a seal of approval merely because they do not witness any bloodshed.

"Mugabe is the world's oldest leader and one of its longest-ruling dictators. He is fixing this election in a more sophisticated fashion than previous ZANU-PF campaigns of beatings, killings and intimidation," the prime minister wrote.

"Mugabe's election-stealing antics have been documented throughout Zimbabwe and beyond. Yet the international community seems apathetic; perhaps Mugabe has been stealing elections for so long the world just rolls its eyes and moves on."

Rallying supporters he calls "soldiers", Mugabe has termed the election a "do or die" contest, suggesting he recognizes that his historical legacy is at stake.

PROTRACTED CRISIS "LIKELY"

Given the irregularities and problems that have dogged the election process, including failure to publish an electronic voters' roll, the result is highly likely to be contested, raising the prospect of another long political stalemate.

"We are prepared to accept the results of a free and fair election but we are not prepared to accept fraud," MDC spokesman Douglas Mwonzora told a news conference.

In 2008, South Africa and other countries in the region brokered a unity government between Mugabe and Tsvangirai to break a deadlock caused by the MDC's withdrawal from a second-round runoff because of the violence and killings.

"A return to protracted political crisis, and possibly extensive violence, is likely," the International Crisis Group, a Brussels-based political risk think-tank, said in a report issued on Monday entitled "Mugabe's Last Stand".

It also criticized the chaotic organization of the election.

Around a third of 63,000 police officers and civil servants allowed to vote two weeks early were unable to cast their ballots because voting materials did not turn up on time.

The existing list of the 6.3 million registered voters has also attracted criticism from the MDC and analysts.

In a study comparing the list to a 2012 census, the Research and Advocacy Group, a non-governmental organization, said young people - the main support base for Tsvangirai - were under-represented, while old people - more likely to be ZANU-PF supporters - were curiously numerous on the roll.

In particular, it cited the presence of more than 116,000 people aged over 100 and said that in almost a third of constituencies there were more registered voters than residents.

The Zimbabwe Electoral Commission has rejected charges the voters' register is a shambles and has accused critics of seeking to discredit the election out of political interests.

But the alleged irregularities, combined with openly partisan security forces and biased state media clearly backing Mugabe's ZANU-PF, have intensified doubts in Western capitals about declaring the elections free and far.

That verdict is crucial to the lifting of Western sanctions against Mugabe and his inner circle, a move that would allow Harare to normalize relations with the International Monetary Fund and World Bank and access the huge amounts of investment needed to rebuild its dilapidated economy.

(Writing by Ed Cropley; Editing by Pascal Fletcher and Paul Taylor)