By Lefteris Karagiannopoulos ATHENS (Reuters) - Keen to clear the decks for its lucrative summer tourist season, Greece is trying to clear thousands of migrants out of its biggest port where they are sleeping rough by persuading them that they are better off in organised reception centres. More than 50,000 migrants have been stranded in Greece because of multiple border closures across the Balkans to the north, sealing off a land corridor to wealthy northwestern Europe used by a million people before them fleeing conflict and deprivation in the Middle East, Africa and Asia. The port of Piraeus is the main gateway to Greece's Aegean islands beloved of tourists - but also for an annual exodus of Greeks from the mainland to celebrate Orthodox Easter.