* Guinea Bissau seeks to turn page on years of instability
* Former finance minister seen as most likely winner
* Fears of post-election violence linger (Adds candidates voting, quotes, PAIGC accusations of intimidation)
By Alberto Dabo
BISSAU, May 18 (Reuters) - Guinea-Bissau's presidential frontrunner sought to dispel fears of potential meddling by the chronically unstable nation's army as voters cast their ballots on Sunday in a high-stakes run-off election meant to draw a line under a 2012 military coup.
Former finance minister Jose Mario Vaz, the candidate of the dominant African Party for the Independence of Guinea and Cape Verde (PAIGC), secured more than 40 percent of the vote in the first round and is considered the strong favourite for victory.
He faces Nuno Gomes Nabiam, the former chair of Bissau's civil aviation agency, who won around 25 percent of the first round vote, comes from the Balanta ethnic group - the country's largest - and is seen as close to the army.
Guinea-Bissau's last vote in 2012 was abandoned after soldiers under army chief Antonio Injai stormed the presidential palace just days before another PAIGC candidate, Carlos Gomes Junior, appeared poised for victory in a scheduled run-off.
In his final speech on Friday ahead of the poll, Vaz called upon the army to remain neutral this time. Speaking to reporters after casting his ballot on Sunday, he attempted to play down rumours of tension between him and the military.
"The army is an integral part of the population of Guinea-Bissau. My relationship with the army is the same as the one I have with the people," he said.
His opponent Nabiam voted in another part of the capital.
"I'm optimistic and sure of my victory," he told journalists.
Weak state institutions, along with its complex maze of islands and unpoliced mangrove creeks, have made the former Portuguese colony a paradise for smugglers of Latin American cocaine destined for Europe.
Since it won its independence in 1974, no elected leader has completed a five-year term in Guinea-Bissau and analysts say donors and regional powers who have been bankrolling the interim administration are frustrated with the recurrent crises.
"These elections will allow Guinea-Bissau to get out of the abyss," said 40-year-old Maria de Fátima Almada Gomes, one the thousands of voters who lined up in front of polling stations well before they opened. They were due to close at 1700 GMT.
No major incidents were reported during last month's first round of voting, in which nearly 90 percent of the registered electorate cast their ballots. The final two weeks of campaigning have mostly taken place peacefully.
However, the PAIGC said on Sunday that the home of one of its senior officials had been the target of an overnight attack by unknown assailants. Fifteen party activists were also attacked in Batfata, 150 km (100 miles) southeast of Bissau, it said.
Both candidates reasserted on Sunday that they would accept the result whoever won. But Marie Gibert, an expert on Guinea-Bissau at Nottingham Trent University, remained sceptical.
"Much will depend on the reaction of their support bases. This is particularly true of the army if Vaz is declared the winner," she told Reuters on Saturday.
Police sources said around 5,000 men made up of police and forces drawn from the Economic Community of West African States regional bloc had been deployed since Friday to provide security until the results are announced, probably later this week.
"We must turn this black page of our history in order to finally decide upon a better future for our people, exhausted by so many years of political, economic and social crisis," said Edmundo Alfama, one of the early voters in Bissau. (Writing by Emma Farge and Joe Bavier; Editing by Alison Williams)