YEREVAN (Reuters) - Former Armenian presidential candidate Raffi Hovannisian began a hunger strike on Sunday calling on President Serzh Sarksyan to resign, accusing him of rigging his re-election last month.

Hovannisian, leader of the opposition Heritage Party took 37 percent of the vote in the February 18 election while Sarksyan was declared the victor with 58.6 percent. Hovannisian objected and filed a complaint with the Constitutional Court.

"This is not just a hunger strike, but a boycott against lies and fraud," Hovannisian told hundreds of his supporters at a rally in the capital Yerevan's central Freedom square.

"If on April 9 (inauguration day) Sarksyan takes his fake oath on the Constitution and the Holy Bible and the Supreme Patriarch...blesses the candidate, who mocks the people, then that will happen over my dead body," he said.

The Constitutional Court is to start considering Hovannisian's complaint on Monday. The central election commission said last month there were no legal violations during the vote that could have influenced the outcome.

Hovannisian, a U.S.-born former foreign minister of the landlocked ex-Soviet republic, submitted 70 complaints to the electoral commission, which responded by saying the documents were based neither on facts nor legal evidence.

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

Since the election, the opposition has held several peaceful rallies to protest against alleged vote rigging.

Investors worry about any signs of instability in the South Caucasus state, where 10 people were killed in violence that followed Sarksyan's initial election in 2008.

Armenia, a country of 3.2 million people, hosts one of Russia's few foreign military bases and is part of a post-Soviet security alliance dominated by Moscow. It borders Iran, Georgia, Turkey and Azerbaijan.

(Reporting by Hasmik Mkrtchyan; Writing by Margarita Antidze; Editing by Mark Heinrich)