More than 20,000 Greeks protested against austerity measures in Athens on Sunday and workers at state-owned utility PPC announced strikes to oppose government plans to sell the company.

In the 19th straight day of demonstrations against a new austerity campaign the Socialist government hopes to pass this month, protesters gathered in front of parliament in Athens' central Syntagma square, shouting "Thieves! Thieves! Thieves!"

Having failed to meet fiscal targets under Greece's 110 billion euro bailout by the European Union and International Monetary Fund, the government wants to raise taxes and slash spending more than planned this year in a bid to avoid default.

It has slated more measures through 2015 to cut the fiscal deficit to 1.1 percent in 2015 from 10.5 percent of gross domestic product last year, including a plan to sell state assets worth 50 billion euros.

A central part of the sell-offs is state energy producer PPC. But labour unions, opposition parties and some backbenchers of Prime Minister George Papandreou's ruling PASOK party oppose selling some Greek state-owned firms.

"We are against the sell-off of the country. We will not close our eyes," the GENOP DEH union of PPC workers said in a news release late on Saturday. "The union has decided to start 48-hour strikes from Monday June 20."

The mid-term austerity plan includes an extra 6.4 billion euros of cuts for this year, almost doubling originally agreed steps that have added to a 5.5 percent economic contraction in the first quarter and driven unemployment over 16 percent.

It has sparked daily protests and caused Papandreou's party to fall behind the conservative opposition in opinion polls. The government aims to begin discussion of the plan this week in parliamentary committees and approve it by the end of June.

Demonstrators have become frustrated with what they say are corrupt politicians who refuse to crack down on wealthy tax-evaders and instead try to solve debt problems by taxing less well-off Greeks.

On Sunday, they jeered at the parliament building and waved banners that read: "We won't leave if they don't leave," and "We don't owe anything, we won't pay anything."

Unions representing hundreds of thousands of workers in the public and private sectors are planning a nationwide strike on Wednesday, June 15.