A leading candidate to become Kyrgyzstan's next president said on Friday that millions would rise to overthrow the leaders of the Central Asian republic if Sunday's election was rigged.

The presidential election threatens to expose a north-south cultural divide in the mainly Muslim country of 5.5 million as it attempts to complete its first experiment with parliamentary democracy after two decades of authoritarian rule.

Instability in Kyrgyzstan concerns the United States and Russia, which operate military air bases there and share concerns over drug trafficking and a spillover of Islamist militancy from nearby Afghanistan.

Kamchibek Tashiyev, a tough-talking former emergencies minister with a strong powerbase in the impoverished south, said his supporters would reject the results of the vote should they believe the elections to be unfair.

In a warning to the front-runner, pro-business northern Prime Minister Almazbek Atambayev, Tashiyev said the authorities should not abuse their administrative resources to sway voters. He vowed to punish any transgressions.

If I lose an honest and transparent election, I will recognise my defeat, he told reporters. If the result of the election is falsified, there will be a huge scandal.

Prime Minister Atambayev, with firm backing from Moscow, is the flag-bearer of reforms last year that turned parliament into the main decision-making body in the gold-rich nation, after its second revolution in five years drove the president into exile.

The reforms watered down the powers of the president.

The new president will replace Roza Otunbayeva, a former ambassador to London and Washington, who became interim president last year after the revolution and steps down at the end of this year.

Her replacement will rule for one six-year term and appoint the defence minister and national security head.

Tashiyev, 43, is an outspoken opponent of the parliamentary model and accused the current authorities of coming to power on the blood of their own citizens, referring to the April 2010 revolution during which 87 people died in the capital Bishkek.

Regional governors, city mayors, village heads and the heads of electoral commissions who falsify the results will be thrown into jail, he said. They are playing with fire.

Tashiyev, one of the leaders of the Ata Zhurt party that polled highest in a close-run parliamentary election last year, has been involved in a punch-up with a member of his own party and manhandled a rival politician inside parliament this year.

If the election is dirty and falsified, we will banish these authorities without bloodshed. And within a short period of time, power will truly belong to the people, he said, adding that his party had stationed 15,000 observers nationwide.

If, after the election, it becomes apparent that the result was falsified, I'm sure the people will not tolerate this. And the people will rise, not in their thousands, tens of thousands or even hundreds of thousands. Millions will rise.

NATIONALIST? NOT ME

Tashiyev and another heavyweight southern candidate, Adakhan Madumarov, can draw on the fervent nationalism of voters in comparatively poor regions of the agrarian south. The north, including the capital Bishkek, is more industrial and russified.

But Tashiyev rejected the idea of a north-south divide as an invention of his opponents, and strongly denied claims that he was a nationalist.

How can you call me a nationalist? My children study in Russia. My wife is Kazakh, and I live among Uzbeks and saved thousands of Uzbek lives, he said, referring to ethnic violence in June last year that killed nearly 500 people in the south.

He said party leaders would not stand idle if people took to the streets. We, the candidates, will not be hiding at home to save our skin, he said. Naturally, we would manage the people ... and we will establish people power.

With a field of 16 candidates, some analysts are forecasting a second-round run-off between Atambayev and whoever comes closest to him.

Analysts say Atambayev has held talks with influential politicians to find a way to bridge any north-south divide by appointing a prime minister and parliamentary speaker from the south if he becomes president.

Victory for Tashiyev would result in the expulsion of the U.S. military from its air base when the current lease expires.

The lease ends in 2014 and, if I become president, I will demand that the current agreement not be extended, he said. He did not mention the Russian air base.

He also suggested he might attempt to reverse the constitutional reforms to restore the power of the presidency. Speaking to Reuters, he said: Who wouldn't want wider powers?

(Writing by Robin Paxton; Edited by Richard Meares)