Greek lawmakers last month approved legislation which allowed PPC to start selling 40 percent of its coal-fired capacity, a requirement of a multi-billion euro bailout accord between Athens and its international lenders.

PPC's board was due to convene on Thursday in Athens to pave the way for the sale of three units and a licence for another one but the meeting was put off after workers occupied the building, the head of PPC's biggest trade union GENOP-DEH said.

"We had occupied the offices since last evening. The workers got in, aiming specifically for the firm's board meeting on such a serious issue not to take place," GENOP-DEH's head George Adamidis told Reuters.

He said the union would meet on Thursday to decide on its next moves.

PPC said in a statement that the board will convene via a teleconference on May 7 to decide on the advisers and other issues of the sale.

PPC needs to take a series of actions to set off the sale process, including securing shareholders' approval to launch an international tender for the divestment.

The utility said on Monday it wanted to invite investors to express initial interest for the units by May this year, aiming aim to conclude the sale by Jan. 17, 2019.

(Reporting by Angeliki Koutantou; editing by David Evans and Jason Neely)