Riot police clashed with workers at Greece's biggest power producer PPC on Thursday as they staged a protest against a new property tax imposed as part of austerity measures to avert the country's bankruptcy.

Some 80 police scuffled with members of the company's labour union GENOP outside the entrance to the building in an Athens suburb. Police detained 15 people, a police spokesman said.

The union is trying to boycott the property tax that PPC has been charged with collecting via Greeks' electricity bills.

We will not back down in our struggle. This fight is about the whole of Greek society. It is about not cutting power to the homes of the poor, the unemployed, the pensioners, Nikos Fotopoulos, head of GENOP, said before being detained.

The fight will continue till the end. This law will become invalid in practice, with the help of all the people.

The protest underscored the resistance conducted by labour unions to the austerity measures the new national unity government must implement to secure the release of loans needed to prevent Greece defaulting on its debts.

Public sector unions representing about half a million workers plan to halt work for two hours later on Thursday in protest against the austerity measures and the government's 2012 draft budget now moving through parliament.

Their protest will include a march to the Portuguese embassy to show solidarity with workers in Portugal, who were staging a general strike on Thursday against tough austerity measures there aimed at meeting EU-imposed budget goals.

Greek private and public sector unions plan a national strike on December 1.

GENOP is one of Greece's most hardline labour unions. Over the past years it has held a number of strikes that have disrupted electricity supplies and scuppered government plans to sell a stake in or find strategic private partners for PPC.

(Reporting by Daphne Papadopoulou, writing by Gareth Jones; Editing by Janet Lawrence)