Greece should know within hours whether it has won crucial support from private creditors to avoid a messy debt default.
A critical bond swap is taking place, with creditors being asked to swap their Greek bonds for new ones, at one-third of the value.
If the last-ditch effort works, Greece will slice $132 billion off its national debt and unlock the international funds it needs to meet its March 20 debt repayment deadline.
If it does not, and Greece defaults, the single currency lurches into foreign territory.
Greece needs more than 66 per cent of its bond holders to agree to the scheme.
There have been signs the deal will find success.
According to local media, more than 75 per cent have indicated they will participate, and the International Institute of Finance (IIF) says an agreement is close at hand.
"I'm optimistic that there's going to be an agreement in the next few hours," said Charles Dallara, managing director of the IIF and chief negotiator for the banks involved in the debt writedown.
The exercise is meant to make repayment of the debt, currently at over 350 billion euros, more sustainable in the immediate future, thereby giving the struggling Greek economy more time to emerge from a five-year trough.
Creditors holding more than half of the total had already agreed to participate in what is essentially a controlled default aimed at calming a crisis that concerns the entire eurozone.
The debt writedown is the biggest ever attempted, overshadowing Argentina's $82 billion default in 2002, the equivalent of 73 billion euros.
