By Lefteris Papadimas ATHENS (Reuters) - Thousands of Greeks rallied on Monday behind a 'No' vote in a referendum called for July 5 on the terms of an aid deal offered by Greece's international creditors, a result that risks pushing Athens out of Europe's currency union. With Greece set to default on a 1.6-billion-euro (£1.14 billion) loan repayment to the International Monetary Fund on Tuesday, at least 20,000 defiant supporters of Alexis Tsipras' left-wing government packed the main avenue in front of parliament, rallying behind his call to reject the package and more austerity. Greece's European partners have said this could see the country slide from the 19-nation euro zone, with unforeseeable consequences for Athens and for Europe's grand project to bind its nations into an unbreakable union by means of a common currency.