* Unknown suspects tried to topple Karamanlis - prosecutors
* Former PM tried to advance energy relations with Russia
By Dina Kyriakidou
ATHENS, March 14 (Reuters) - A Greek prosecutor pressed charges of treason and trying to destabilise the country against unknown suspects on Wednesday over an alleged 2008 plot to overthrow the then-conservative prime minister, Costas Karamanlis.
Court officials said the plan was hatched to thwart efforts by Karamanlis efforts to improve energy relations with Russia and included overthrowing and even killing the premier. It was not clear who was behind it.
"An investigative prosecutor will now try to locate the persons who devised and organised the plan," said a court official who requested anonymity.
The official said the charges were prompted by press reports about a Russian document revealing the plot and came after testimony from Greek secret service agents, police and Karamanlis' security guards.
Karamanlis, who lost elections to the socialists in 2009 in the wake of a series of financial and political scandals, had improved ties with Moscow and advanced plans for a major pipeline to bring Russian gas to western Europe.
There was no immediate reaction from the former prime minister's office. The conservative New Democracy party would not comment on the charges.
Karamanlis has been on the sidelines of conservative politics since his crushing defeat, refusing to comment on his government's failings. Under the leadership of Antonis Samaras, New Democracy has recovered some ground and is leading opinion polls ahead of elections expected in late April or May.
The nephew of the late Greek statesman Constantine Karamanlis, the ex-premier has not made public how he intends to participate in the upcoming election.
His 2004-2009 government has been largely blamed for failing to avert the debt crisis now shaking Greece and the euro zone.
Court officials said the prosecutor also concluded that some of the violence that erupted after the police shooting of a teenager in December 2008 was part of the plot aimed at destabilising Greece.
The boy's killing and the weeks of violence that gripped the country shook Karamanlis' government and led to snap elections the following year.
The criminal investigation by prosecutor Nikos Ornerakis was spurred by press reports last year, led by the Epikaira news magazine, that an assassination plot code-named "Pythia" was organised by a unnamed country allied to Greece to prevent it from clinching pipeline deals with Russia.
"I remember the PM's security was beefed up at the time. It was taken quite seriously," a former aide to Karamanlis told Reuters on condition of anonymity.
Karamanlis pursued closer cooperation with Russia on energy issues, annoying Greece's western allies.
He supported the South Stream gas pipeline, which will bring Russian natural gas to Greece and western Europe and rivals the Nabucco, the U.S.-backed pipeline that will convey gas from central Asia to the West. (Editing by Mark Heinrich)