By Hasmik Mkrtchyan

YEREVAN (Reuters) - The runner-up in Armenia's presidential election said on Monday he might challenge the official result that gave Serzh Sarksyan a new five-year term and triggered street protests.

Opposition leader Raffi Hovannisian got 37 percent of votes cast in the February 18 election in the nation of 3.2 million, Moscow's closest ally in the south Caucasus, nestled between Russia, Turkey, Iran and energy-rich Central Asia.

Hovannisian and his Heritage Party say the vote was rigged in favor of Sarksyan, who won 58.6 percent. The opposition have twice held protests drawing several thousand people in the capital Yerevan since the election.

"As of today we have five days to challenge the clearly illegal and anti-Armenian result of this election," Hovannisian, a U.S.-born former foreign minister of the landlocked ex-Soviet republic, told a news conference.

"Many say it's not worth it. We will discuss it. I do not rule out that by March 2 we will turn to the Constitutional Court," Hovannisian said, referring to an official challenge. He repeated that he considers himself the real winner of the race.

Armenia's election commission also said on Monday there were no legal violations during the vote that could impact results.

The protests on Wednesday and Friday were peaceful. Foreign governments and investors worry about signs of instability in Armenia, where 10 people were killed in violence that followed Sarksyan's first election victory in 2008.

International election monitors said last week's poll was an improvement from previous ones but it still lacked real competition after some of Sarksyan's adversaries decided not to run, fearing the results would be skewed.

Armenia, which hosts one of Russia's few foreign military bases and is part of a post-Soviet security alliance dominated by Moscow, is locked in a deadly dispute with neighboring Azerbaijan over the mountainous Nagorno-Karabakh region.

Nagorno-Karabakh is an ethnic Armenian-majority enclave inside Azerbaijan, which Armenia-backed rebels wrested from Azeri troops in a war that killed some 30,000 people before a 1994 ceasefire. Skirmishes still kill troops on both sides.

(Reporting by Hasmik Mkrtychan, Writing by Gabriela Baczynska, Editing by Steve Gutterman and Michael Roddy)