ATHENS - The leader of Greece's conservative opposition has asked Prime Minister George Papandreou to stand down and agree to a commonly accepted new head of government, a source from the New Democracy party told Reuters."(New Democracy leader) Mr. Samaras asked Papandreou to resign and for a commonly accepted person to be appointed as the new head of government," said the party source, who did not want to be named.Papandreou's office was not immediately available for comment.