ATHENS (Reuters) - Greek conservative opposition leader Antonis Samaras will hold talks with the country's president on Sunday, the president's office said, after opposition parties condemned the government's call for a referendum. Samaras, head of the New Democracy conservatives, earlier this week said he would be willing to participate in a cross-party government to pull Greece back from the brink of bankruptcy. (Reporting by Karolina Tagaris, Writing by Deepa Babington)