* Tiny West African state a hub for cocaine trafficking
* Election marks chance for stability, but could go wrong
* Opposition figure says can't accept ruling party win (Updates with polls closing, counting starting)
By Richard Valdmanis and Alberto Dabo
BISSAU, March 18 Guinea Bissau held a peaceful vote for a new president on Sunday in an election meant to steer the coup-prone West African state towards stability, but which could instead prolong its history of turmoil if the results are contested.
International partners are keen to see the tiny nation, whose president died in January after a long illness, clamp down on rampant drugs trafficking that has made it the main African transit point for South American cocaine bound for Europe.
"Guinea Bissau needs a new leader, one who can bring the country forward, who has the ability to restore peace, stability and unity," said Maria Evarista Sousa, 55, after voting in a dusty suburb of the capital Bissau.
The vote will be seen as the latest test for democracy in a region that has seen a recent flurry of troubled elections, including Ivory Coast's in 2010 that sparked a civil war, and Senegal's, which triggered deadly street violence. Senegal is holding a second-round run-off on March 25.
In Guinea Bissau, former prime minister and ruling party candidate Carlos Gomes Junior is up against main rivals Manuel Sherifo Nhamadjo, who dropped out of the ruling party to run as an independent, and Kumba Yala, an ex-president who shares the Balanta ethnicity with a quarter of the population and most of the army.
The leading candidates have promised to make fighting drugs a priority, but observers say doing so will require more foreign aid as well as alliances with a military believed by many to be complicit in the drugs trade.
An estimated 800-1,000 kg of cocaine are flown into Guinea Bissau every night, according to a leaked 2009 U.S. diplomatic cable, along with an unknown amount ferried by sea into the maze of mangrove-lined islands that make up much of its coast.
Election observers said voting appeared to pass off smoothly in the country of 1.6 million people and that counting was well underway. The national electoral commission has 10 days to publicise the poll's results, but could start posting partial figures sooner.
The result may prove contentious, however: opposition politicians have already accused Gomes Junior of undermining the fairness of the poll after his government refused to update a voter register from 2008, leaving more than 100,000 people off the list.
"We will never accept the fabrication of results," candidate Yala said at his home in Bissau, a sleepy city of crumbling buildings near a river that drains into the Atlantic. He said he would seek to have the results annulled if Gomes Junior wins.
Any dispute over the election outcome could raise chances of the military stepping in, as it has done repeatedly since the country's independence from Portugal in 1974, with coups, arrests and political assassinations.
Whoever wins will have a huge task to develop the country, whose main official export is cashew nuts.
An ordinary Bissau Guinean lives on less than $2 a day. A combination of military meddling and health problems has prevented any president from serving a full term since multi-party politics began in 1994.
Gomes Junior, a former banker reputed to be Bissau's richest man, served for years as prime minister under presidents Joao Bernardo Vieira, assassinated in 2009, and Malam Bacai Sanha, who died in Paris in January after a long illness.
"I believe I will win in the first round," he said after casting his ballot alongside his family at an open-air voting station in the centre of Bissau, as a couple of dozen supporters chanted "victory".
Known locally as Cadogo - an abbreviation of his father's name, Carlos Domingos Gomes - he has won tacit international support during his time in office for economic reforms and speaking out against drug smuggling.
The Paris Club cancelled $283 million in debt last May, the IMF has provided a line of credit, and Angola has contributed money and troops for army reform while also planning a bauxite mining and port construction project.
But his rivals say he has been weakened by a split in the ruling PAIGC party over his candidacy - which has become controversial because of the turmoil and increased drug running that accompanied his time in government.
Manuel Sherifo Nhamadjo, an influential member of the party forged during Guinea Bissau's struggle for independence, dropped out to run against Gomes Junior as an independent in a move that could split the traditional PAIGC vote.
"We must work to establish peace in Guinea Bissau so that we can develop the country," Nhamadjo said on Sunday, after voting in the shade of a cashew tree on the city's outskirts.
Results are expected within a week. If no candidate wins an outright majority, a run-off will probably be held in April. (Writing by Richard Valdmanis; Editing by Andrew Osborn)