By George Georgiopoulos

ATHENS, June 5 (Reuters) - Tens of thousands Greeks rallied in central Athens on Sunday to denounce politicians, bankers and tax dodgers, as the government prepared to inflict another bout of austerity demanded by its international lenders.

"Thieves - hustlers - bankers," read one banner as more than 50,000 people packed the main Syntagma square outside parliament to vent their frustration over rising joblessness as austerity bites, blaming the crisis on political corruption.

Turnout was the biggest so far in a series of 12 nightly rallies on the square inspired by Spain's protest movement.

Amidst a sea of splayed hands waved at the parliament building -- an offensive gesture for Greeks -- one demonstrator raised a placard reading "Bravo Yemen", whose president underwent surgery in Saudi Arabia for injuries suffered in a rocket attack on his palace.

Police put the crowd at 50,000 by mid-evening, but numbers continued to grow as dusk fell over the Greek capital. Another banner drew comparisons with rallies early this year in central Cairo which ousted Egyptian President Hosni Mubarak. "From Tahrir Square to Syntagma Square, we support you!" it said.

The cabinet of Prime Minister George Papandreou is due to discuss on Monday an economic plan, which a senior government official said would impose 6.4 billion euros of budget measures this year alone, on top of austerity already imposed under Greece's original international bailout agreed last year.

The medium-term plan includes tax increases while the international lenders are pushing for a crackdown on widespread tax evasion. The black economy is thought to be around 20-30 percent of gross domestic product.

TAX CHEATS

"Instead of going after tax cheats, they are raising taxes and cutting working people's pay," said Yannis Mylonakos, 34, who lost his job at an advertising agency.

As Greece battles to avoid defaulting on its debt, which totals about 340 billion euros, unemployment has soared to almost 16 percent.

The extra austerity is the price for a new bailout agreed with the European Union and International Monetary Fund to replace the old one, which has proved overoptimistic in assuming Greece could resume borrowing commercially early next year.

The Syntagma Square rallies, organised through Facebook, so far have been peaceful, more festive and less politically motivated than traditional labour union protest rallies.

Protesters from all over Greece on the square rejected the austerity policies to cut the budget deficit that led to layoffs, wage and pension cuts and a heavier tax burden.

"You got the disease, we got the solution -- revolution," read one banner. "We are not commodities in the hands of bankers and politicians."

Protesters also gathered in Greece's second city of Thessaloniki.

Organisers say they are determined to continue indefinitely as the number of people joining the Facebook group "angry at Syntagma" is growing. Some are camping on the spot, with about 30 tents on the square.

"We don't owe, we don't sell, we don't pay," read a banner hung on the square's lamp posts. Students, pensioners, young couples with their children and immigrants, were among those gathered on the square over the past week.

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