The bond buyback is central to the efforts of its foreign lenders to put Greece's debt back on sustainable footing, and its success will pave the way for the country to get long-delayed funding to avoid bankruptcy.

Since plans for the buyback were announced last week, questions have swirled about whether it will tempt enough bondholders to cut Greek debt by a net 20 billion euros -- the target set by euro zone finance ministers and the International Monetary Fund.

The buyback will be conducted through a modified Dutch auction that allows it to introduce an element of competition among investors to get the best price.

Greece set a price range to buy back each of its 20 series of outstanding bonds with a spread of two percentage points - from a minimum of 30.2 to 38.1 percent and a maximum of 32.2 to 40.1 percent depending on the bond maturities.

In such an auction, if a bondholder tries to get a price close to the upper limit there is a risk he or she may be left out if the buyback amount is filled at lower prices. There will be one settlement price for each series of bonds.

Greek bonds eligible under the buy-back ranged from 25.15 to 34.41 cents in the euro at the close of trading on November 23, Reuters data showed.

Athens said it would not spend more than 10 billion euros on the buyback. Investors must declare their interest by December 7 and the expected settlement date is December 17.

Euro zone officials said the bloc hoped Greece would be able to repurchase at least 40 billion euros of its own bonds.

Athens unveiled the structure of the buyback before a meeting of euro zone finance ministers, at which Greek Finance Minister Yannis Stournaras will brief his counterparts.

Greece's lenders agreed last week that the bonds, which have a nominal value of 63 billion euros, would not be purchased for more than the closing price on that date. The offer goes in theory also to holders of about 4 billion euros of old Greek bonds, who refused to take part in a debt cut scheme in March.

A Reuters calculator on the buy-back shows that if the buy-back price was set at the November 23 closing prices, even a 50 percent participation rate would be enough for a successful deal - in this instance, Athens would have to spend just 8.7 billion euros to buy back debt worth 31.5 billion euros.

For Athens to spend 10 billion euros, it would have to buy back around 60 percent of the outstanding bonds. This could save Greece 39 billion euros gross on the face value of the bonds and the interest payments due on them.

(Additional reporting by Karolina Tagaris.; Writing by Deepa Babington, editing by Mike Peacock)

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