BISHKEK (Reuters) - Kyrgyzstan votes in a presidential election on Sunday that is being closely fought between a protege of the outgoing president and an opposition leader, testing stability in a country where two previous leaders were ousted in violent riots. The mainly Muslim ex-Soviet state of six million people stands out from its neighbors, mostly run by autocrats, in having a boisterous democracy that produces sometimes chaotic changes of leadership. Serving President Almazbek Atambayev has to step down because of term limits, and is backing Sooronbai Jeenbekov, a 58-year-old member of Atambayev s Social Democratic Party, to succeed him. He is facing strong opposition from Omurbek Babanov, 47, who heads the biggest opposition party, Respublika-Ata Zhurt (Fatherland). The most likely outcome is that neither Babanov nor Jeenbekov will win outright on Sunday, setting up a second round run-off, according to the U.S.-based International Republican Institute, which has conducted its own polling.  Both leading candidates differ little on the main points of policy: a secular state, hewing close to Russia in a region where Moscow vies for influence with the United States and China. However, they represent rival interest groups and clans inside Kyrgyzstan that are fighting for access to power and state resources. In a September poll by Coalition for Democracy and Civil Society, a Western-backed NGO, 38.7 percent of respondents said they expected Babanov to win and 41.2 percent picked Jeenbekov. Eleven more candidates are running in the vote. A candidate needs more than 50 percent for an outright win. Opposition candidate Babanov is a wealthy businessman with a charismatic style, while Jeenbekov is a former livestock technician turned bureaucrat. Both say they will maintain strong ties to Moscow.  Kyrgyzstan hosts a Russian military airbase. Remittances from hundreds of thousands of migrant laborers working in Russia are a vital source of Kyrgyzstan s foreign currency revenue. The campaign has featured allegations of coup plots and corrupt practices, hinting at the fragile political stability that has dogged Kyrgyzstan since a revolution in 2005 that ousted long-standing leader Askar Akayev.   I am going to start my fight against corruption with you,  Jeenbekov told opposition candidate Babanov in a televised debate, in which he also called his younger opponent  a chick . The Kyrgyz government detained one of Babanov s supporters on Sept. 30, charging him with plotting a violent coup during the vote. It has also accused several Babanov campaign workers of planning to bribe voters. Babanov has denied any wrongdoing and dismissed the charges against his supporters as dirty election tactics. Atambayev s government last month accused Kazakhstan, a bigger and wealthier neighbor which is also close to Russia, of interfering in the election after Kazakh President Nursultan Nazarbayev met Babanov and appeared to endorse him.   Kazakhstan has denied backing any candidates, and, citing security concerns, introduced tighter checks on the Kyrgyz border this week. 