* Guinea Bissau seeks to turn page on years of instability
* Former finance minister seen as most likely winner
* Fears of post-election violence linger (Updates with polling stations closing, adds turnout and observer comments)
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 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 the civil aviation agency, who won around 25 percent of the first round. He comes from the Balanta ethnic group - the country's largest - and is seen as close to the army.
Voting closed on schedule around 1700 GMT at polling stations visited by Reuters in the capital Bissau, and elections workers immediately began counting ballots.
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.
Vaz has 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 with 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.
Voter turnout was between 60 and 65 percent by 1530 GMT, the elections commission said, indicating a drop in participation from the nearly 90 percent recorded in the first round.
NO MAJOR INCIDENTS
Weak state institutions, along with its 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, who lined up in front of polling stations well before they opened.
A first round of voting last month was largely praised by observers, and 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.
The European Union said it registered the PAIGC's complaints among others but overall the vote was peaceful.
"Speaking separately with the two candidates, I urged them to wait for the proclamation of results by the elections commission before declaring victory," Krzystop Lisek, the EU's chief elections observer, said.
Both Vaz and Nabiam restated on Sunday they would accept the result whoever won. (Writing by Emma Farge and Joe Bavier; Editing by Alison Williams and Cynthia Osterman)