* Offer extended to holders of 1.1 bln euro securities
* Piraeus wants to boost capital base
* Offer starts on Thursday, ends on Nov. 4 (Add more detail, background)
ATHENS, Oct 15 (Reuters) - Piraeus Bank on Thursday launched an exchange offer for bondholders aimed at boosting its capital which has been hit by tough economic conditions in Greece.
Holders of Piraeus Bank securities with a value of 1.1 billion euros ($1.26 billion) can exchange any or all of them for non-transferable receipts, Piraeus said. Investors will subsequently be offered either cash or equity in exchange for those receipts.
Piraeus said the exchange offer was being made to strengthen the bank's capital base.
The European Central Bank is testing Greece's four largest banks - National Bank of Greece, Piraeus, Eurobank and Alpha - to determine how much capital they need after a downturn in the Greek economy.
Under an international bailout agreed last summer, Greece is set to receive up to 25 billion euros of public money to recapitalise its banks, many of which are partly state-owned.
Piraeus said under the exchange offer, holders of three separate series of bonds - perpetual securities of 200 million euros, securities of 400 million euros due 2016 and securities of 500 million euros due March 2017 - can accept the offer, which begins on Thursday and ends on Nov. 4. ($1 = 0.8730 euros) (Reporting by Angeliki Koutantou; editing by Jason Neely and Jane Merriman)