* Public, private sector unions to strike May 20

* Unions threaten to step up protests

* Say resolved to block “disaster” austerity recipes

(Updates with new strike date, details)

ATHENS, May 12 (Reuters) - Greek workers on Wednesday called a 24-hour general strike for May 20, the latest in a series of protests against planned pension cuts linked to an international 110-billion-euro ($139.7 billion) bailout for Greece.

Public and private sector unions had initially called a strike for May 19, but decided later on Wednesday to move the walkout a day later, saying they did not want to hinder national university entry tests on the same day.

Unionists have vowed to stop the pension reform, a key condition in a financial rescue plan for Greece agreed last week with the euro zone and the International Monetary Fund (IMF).

The country’s socialist government unveiled a draft law on Monday that raises the average retirement age and pares back benefits to plug the pension system’s ballooning deficits.

“The IMF will not stop thirsting for workers’ blood,” said Yannis Panagopoulos, chairman of Greece’s main private sector labour union GSEE. “Its recipes are a disaster and the government must turn them down.”

The walkout is expected to ground flights to and from Greece, disrupt transport and shut down ministries and public services across the country. The unions will convene after the strike to decide on future action.

Greek workers last went on strike on May 5, when protesters threw petrol bombs at a bank in Athens, killing three employees, in the worst violence the country saw since riots in 2008.

Civil servants and public sector employees, who have vowed to step up protests, will march to parliament late on Wednesday to protest against austerity.

Greece submitted this week a formal request for the first tranche of the EU/IMF multi-billion euro aid, seeking money to refinance an 8.5-billion-euro bond maturing on May 19. (Reporting by Renee Maltezou; Editing by..)