ATHENS, Nov 15 (Reuters) - Demand for a 20% stake in Greece's National Bank (NBG) held by the country's bank bailout fund was covered by more than six times on Tuesday, in the first day of a book-building process, a source with knowledge of the matter told Reuters.

The shares in NBG, Greece's second-largest bank by market value, are being sold via a public offering and a private placement from Nov. 14-16 at between 5 and 5.44 euros per share.

The value of the 20% stake is estimated at about 1 billion euros ($1.09 billion).

"Demand covered 6.2 times the shares offered," the source involved in the process said on condition of anonymity.

Greece on Monday announced plans to sell the 20% stake in NBG after selling a 9% stake in Alpha Bank to UniCredit as it looks to divest from the country's lenders.

HFSF currently holds a 40.4% stake in NBG and a 27% holding in Piraeus Bank, Greece's third-largest lender.

A source told Reuters on Monday that out of the 20% stake, 17% would be offered to funds and 3% to retail investors. If there was strong demand, HFSF would consider increasing the stake on offer to 22%.

JP Morgan, Goldman Sachs, Morgan Stanley and UBS have been appointed as global coordinators of the sale. ($1 = 0.9189 euros) (Reporting by Lefteris Papadimas, Editing by Louise Heavens)