* Philanthropist Kiska draws support from centre-right
* Many voters don't want PM Fico's party to win more power
* Polling ends Saturday evening, results due overnight
By Jan Lopatka
PRAGUE, March 29 (Reuters) - Slovakia's political heavyweight Prime Minister Robert Fico faces the threat of his biggest defeat at the ballot box from an underdog philanthropist in presidential election runoff on Saturday.
Voters in the central European country gave the centre-left leader a disappointingly lukewarm endorsement in the first round of voting two weeks ago amid fear the 49-year-old would amass too much power, which some see as unhealthy for democratic checks and balances.
In the second round, bookmakers give an edge to political newcomer Andrej Kiska, a businessman turned philanthropist who is riding on the wave of anti-Fico sentiment among right-wing voters as well as distrust in mainstream political parties suspected of complicity in graft scandals.
A Fico victory would give his centre-left Smer party full control of all the main power centres in the euro zone country of 5.5 million, even if the Slovak constitution does not grant the president himself a huge political role.
Fico, who has led the country since sweeping a parliamentary election in 2012, won 28 percent of the vote to Kiska's 24 percent on Saturday, a smaller margin than opinion polls had indicated. There have been no fresh polls since the first round.
The president has the power to name or approve some of the main figures in the country's prosecution and judicial branches, and this right has led to political clashes in the past.
Rule of law is a key concern for Slovaks as well as foreign investors, the source of economic growth in the past decade.
Slovakia has lured big foreign manufacturers such as car makers Kia and Volkswagen, which have helped keep growth at decent levels even as others in central Europe slipped into recession amid the euro zone crisis.
Campaigning has turned tough in the final weeks. Fico accused Kiska of usury in consumer lending firms he used to own, for which Kiska filed a criminal complaint against his opponent.
Fico has also accused Kiska, an independent candidate with no party of his own, of being close to Church of Scientology, founded by science fiction writer L. Ron Hubbard in 1954. Scientology is not registered in mostly Catholic Slovakia.
Kiska denied any close links to Scientology, which critics describe as a cult that harasses people who try to quit.
Kiska, 51, has campaigned on the argument that Slovenia needs a counterweight to Fico's Smer party, which has one-party cabinet rule and controls the majority in parliament.
"Smer" is the Slovak word for "direction" and the party's political orientation is basically social democratic.
"If the president is to represent people, he cannot be the extended hand of a political party," Kiska said in the final television debate earlier this week.
The candidates are close on Slovakia's foreign policy, which keeps the country firmly in the EU's pro-integration camp.
Fico would have to give up his post of prime minister if he wins, but his party would replace him with a Smer nominee. A possible choice to succeed him is Robert Kalinak, the interior minister and longtime member of the party that Fico founded.
Fico, 49, took Slovakia into the euro zone in 2009 and has kept the country friendly to investors despite levying extra taxes on banks and utilities.
Polls open at 7 am (0600 GMT) and close at 10 pm (2100 GMT) on Saturday, and results are expected overnight. (Reporting by Jan Lopatka; Editing by Tom Heneghan)