ATHENS (Reuters) - Former Prime Minister Antonis Samaras announced he was stepping down as leader of Greece's conservative New Democracy opposition party after a strong vote for the 'No' position backed by the leftwing government in Sunday's referendum. "Our party needs a new start. As of today, I'm resigning from the leadership of New Democracy," Samaras said in a televised statement. (Reporting by Michele Kambas and Karolina Tagaris)