Greece was to submit a detailed bailout request to its eurozone partners Thursday in a last-ditch effort to save its collapsing economy and its membership in the European single currency.

In a surprise concession, EU President Donald Tusk said that if Greece delivered, its creditors would have to drop their steadfast refusal to look at ways at managing the country's massive $350 billion debt mountain. "The realistic proposal from Greece will have to be matched by an equally realistic proposal on debt sustainability from the creditors," Tusk said. "Only then will we have a win-win situation."

Prime Minister Alexis Tsipras on Wednesday vowed "concrete proposals, credible reforms, for a fair and viable solution" in his bailout request for billions of euros, the third plan Greece has asked for in five years of deep financial crisis. Tsipras was huddled with advisers, pulling together a package of reforms that is expected to include much tougher measures than those included in a previous plan from creditors that was rejected by Greeks in a referendum on Sunday.

But having won wide public support at the referendum and the subsequent backing of opposition parties, an emboldened Tsipras is expected to have an easier time facing down any resistance at home, allowing him to focus to appeasing creditors.

Other eurozone nations, led by Germany, are skeptical about Greece meeting austerity conditions attached to the loans, especially after Greeks in a weekend referendum backed Tsipras's past rejection of tough terms on its last bailout.

These nations refuse to study debt relief or restructuring for Greece until it shows a commitment to reforms.

Tusk has called the next few days "really and truly the final wake-up call for Greece and for us." Leaders of the 28-nation European Union are to hold a summit on Sunday billed as the "final deadline" for a deal.

While Europe has no provision under its treaties to force a country out of what is meant to be an "irreversible" monetary union, some legal advisers say it could be made to happen by kicking an errant state out of the European Union.

A demonstration of Greeks calling for their country to remain in Europe was to take place in central Athens late Thursday.