* Ordinary Greeks back new Greek PM but oppose austerity
* Thousands to take part in annual" November 17" march
* Unions warn of more strikes
By Renee Maltezou
ATHENS, Nov 15 (Reuters) - Thousands are expected to rally in Athens on Thursday, threatening Lucas Papademos' political honeymoon as Greece's new prime minister, a week after former rivals appointed him with a mission to save the country from financial ruin.
Polls may show three-quarters of Greeks back his premiership after decades of Socialist PASOK party and conservative New Democracy rule, blamed for leading Greece to the brink of bankruptcy.
But ordinary Greeks say their support comes with a condition -- no more belt-tightening that hits the poor. They are vowing to ram the point home by marching in their thousands in an annual rally commemorating the Nov 17 student uprising in 1973.
"We expect massive participation as rage and anger has been dwelling in people for so long," said Mary Bossis, international security professor at the University of Piraeus. "It is very possible that some groups will misbehave."
The size and ferocity of the rally, the first major protest in almost a month, will signal just how bitterly a restive public will fight further tax rises and spending cuts that international lenders demand as a condition for the bailout.
"The message on November 17 will be that Greeks can't take it anymore," said general secretary of private sector union GSEE Nikos Kioutsoukis. "People will take to the streets again when they dip into their pockets and feel that they are empty."
The former banker's coalition government has a mandate of around three months to secure a short-term loan needed to stop default and then get parliament's backing for a 130 billion euro bailout that should shore up state finances until 2014.
Many Greeks said they were prepared to cut Papademos some slack, but not for long.
"We'll give Papademos time, but he has promised not to take any more measures that burden the same people. I want to see whether this government can actually keep its promises," said Maria Paraskevopoulou, a 23-year old practising lawyer.
Austerity remains a bitter pill to swallow for a mass of middle class Greeks who have diligently paid their taxes and social contributions and blame the crisis on a corrupt political elite and rich tax evaders.
"We want just measures. We would be willing to pay even more if tax evaders and politicians were punished," said Hara Kappa, a 67-year old retired university professor.
Some 17,000 workers, students and pensioners took part last year in the same march that marks the anniversary of a student revolt that helped topple the 1967-1974 military junta. But last month anti-austerity protests swelled to 100,000 people.
The streets of Athens have enjoyed a rare calm since socialist former Prime Minister Papandreou called a referendum on the second bailout deal, but paid for it with his political life when he agreed to step down in exchange for a vote of confidence in his government.
Unions suspended further industrial action for the first time in two years as they watched their bitterest opponent fall. But an almost unavoidable stepping up of austerity measures to plug budget holes is expected to end the uneasy truce.
"Further austerity measures -- which will be without doubt coming -- are likely to trigger a new wave of street protests," said Blanka Kolenikova, Europe analyst for IHS Global Insight.
Shops will shut down and traffic will to come to a halt as the march wends its way to the U.S. embassy -- which is blamed for supporting the junta 40 years ago. Authorities will flood the streets with 7,000 police to prevent violence by hooded youths which has marred previous rallies.
Papademos, vice president at the European Central Bank until last year, has not dealt with Greece's aggressive labour unions before. The main GSEE and ADEDY grouping, which represent half the country's 4 million workers, say Greece is in a "death spiral".
Unions plan to stage more strikes to coincide with a parliamentary vote for Greece's 2012 budget, which is due to take place before the end of the year.
"Thursday's march will be a warning," said Ilias Iliopoulos public sector union ADEDY general secretary. "If they continue with austerity, we'll step up protests."