* Sarksyan had big lead in polls before vote
* Incumbent had promised stability
* Campaign tainted by assassination attempt on rival (Adds quotes, background)
By Hasmik Mkrtchyan
YEREVAN, Feb 18 (Reuters) - Armenian President Serzh Sarksyan easily won a new five-year term on Monday, an exit poll showed, in an election overshadowed by a lack of serious opposition and an attempt to kill a minor candidate.
The opposition Heritage Party complained that many ballots cast for opposition parties had been thrown out, but did not say whether it would challenge what appeared to be a runaway victory for Sarksyan.
The president, 58, has vowed to sustain economic recovery in his landlocked South Caucasus country and said before the vote he would ensure stability after years of war and upheaval.
The continued peace of a region where pipelines take Caspian oil and natural gas to Europe is a concern for foreign investors and neighbours, especially as relations are fraught with fellow former Soviet republic Azerbaijan.
"I voted for the future of Armenia, for the security of Armenia, for the security of our citizens," Sarksyan said as he cast his vote at a polling station in the capital, Yerevan.
The exit poll by Baltic Surveys/The Gallup Organization, reported by Armenian television, put Sarksyan on 58 percent of votes cast. His nearest rival, U.S.-born Raffi Hovannisian, trailed on 32 percent.
The result, if confirmed, will strengthen Sarksyan's hold on Armenia, which borders Iran, Georgia, Turkey and Azerbaijan, following his Republican Party's victory in a parliamentary election last year.
The immediate reaction on the streets of Yerevan was muted as voters awaited official results. Opinion polls had long predicted Sarksyan would win after his biggest potential challengers opted not to run.
"I voted for Serzh Sarksyan to give him a chance and I will demand that he fulfils everything correctly as he promised," said Mariana, a housewife who declined to give her last name.
The exit poll was carried out among 19,130 voters at 122 polling stations. The central Election Committee said more than 60 percent of the country's 2.5 million eligible voters cast their ballot.
HOPES OF STABILITY
"This is a victory of our society, in as much as the process of the election and the voting proved that democratic processes are irreversible in Armenia," said parliamentary vice-speaker Eduard Sharmazanov, a member of Sarksyan's party.
Heritage Party leader Hovset Khurshudyan disagreed, saying there had been "many violations", but election observers will not issue their findings until Tuesday.
Sarksyan's promises of economic recovery resonate with voters in the country of 3.2 million, where more than 30 percent live below the poverty line. The average monthly wage is about $300 and unemployment was 16 percent last year.
"Sarksyan promotes the improvement of an educated society, which is a guarantee of Armenia's future," said Artak Avetsyan, 31, a teacher who came to cast his ballot for the incumbent.
But durable economic growth will be difficult while its borders with Azerbaijan and Turkey remain closed. Turkey shut the border in 1993 in solidarity with its ethnic kin in Azerbaijan.
Most regional pipeline projects between growing regional power Turkey and the oil and gas-producing Azerbaijan isolate Armenia, making it more dependent on ties with its Soviet-era master Moscow, which maintains a military base on Armenian soil.
Questions also linger over security in the nation locked in a dispute with Azerbaijan over Nagorno-Karabakh, an ethnic Armenian-majority enclave inside Azerbaijan over which Armenians and Azeris fought a war in the 1990s. Sarksyan, like many of his generation, is a veteran of that war.
Sarksyan has accused Azerbaijan of threatening a new conflict. Baku denies it is the aggressor in the conflict and says Armenia should hand back control of Nagorno-Karabakh.
Election observers before the vote expressed concern over the democratic credentials of the vote as none of Sarksyan's serious opposition rivals chose to stand.
The last presidential election, in 2008, was marred by clashes in which 10 people were killed, but there were no immediate reports of violence on Monday.
Officials from the Parliamentary Assembly of the Council of Europe (PACE) reported public apathy and a lack of confidence in the electoral process when they visited the country in January.
Domestic security concerns were underlined by an attack on one of the candidates, Paruyr Hayrikyan, 63, an outsider in the election who was shot in the shoulder on Jan. 31.
Another dark horse in the race, Andrias Ghukasyan, went on hunger strike at the start of the campaign to press for Sarksyan's candidacy to be annulled and for international observers to boycott the vote.
A third candidate, Arman Melikyan, had said he would not vote because he believes the election would be skewed in Sarksyan's favour. Other potential rivals said they chose not to take part in the race for similar reasons.
Final results were expected late on Tuesday. (Additional reporting by Thomas Grove; Writing by Timothy Heritage and Thomas Grove; Editing by Tom Pfeiffer)