By Hasmik Mkrtchyan

YEREVAN (Reuters) - One unsuccessful Armenian presidential candidate was arrested and charged on Wednesday with the attempted assassination of another, authorities said, in an incident that marred the February 18 election in the former Soviet republic.

Opposition politician Paruyr Hayrikyan was shot in the shoulder on January 31. He declined to seek a postponement and went on to receive less than 2 percent of the votes in the election in which President Serzh Sarksyan won a second term.

The Armenian security service said it had arrested Vardan Sedrakyan who was charged with ordering the assassination of his rival. He faces up to 20 years in prison if convicted.

"Sedrakyan does not consider himself guilty, refuses to give any testimonies and intends to appeal the charges in the court," Sedrakyan's lawyer, Alexander Sirunyan, told website News.am. His client won less than 1 percent of votes in the election.

The shooting added to concerns over stability in the South Caucasus region, a key transit route for Caspian energy resources to Europe, where Armenia and its neighbor Azerbaijan are locked in a territorial dispute.

Supporters of Sarksyan's closest challenger in the race, opposition leader Raffi Hovannisian, have staged several peaceful protests in Yerevan since the vote, which they say was rigged.

His Heritage Party has asked the Constitutional Court to the throw out the official result and name Hovannisian the winner. Armenia's election commission has said there were no violations that could have affected results.

(Writing by Margarita Antidze; Editing by Sophie Hares)