ATHENS (AFP) – Greek Prime Minister George Papandreou on Wednesday invited the main opposition into government to deal with the debt crisis, as thousands ringed parliament against new austerity cuts.

Papandreou sounded out opposition conservative leader Antonis Samaras on a unity government to secure backing for new controversial reforms required to secure another debt bailout for Greece, state television NET said.

Government officials could not be reached for comment but Papandreou was expected to make a televised address later in the day.

The conservative party is expected to call on Papandreou to step down, news reports said.

Some 40,000 people, according to media estimates, joined a mass protest in Athens and ringed parliament in a protest timed to coincide with a crippling general strike, the third this year against the austerity cuts.

Police fired tear gas as clashes with protestors left at least 12 injured, television and the police said.

Another 20,000 people gathered in the main northern city of Thessaloniki, police said.

The show of force in Athens was organised by a protest group that has peacefully occupied the central Syntagma Square, where parliament is located, for weeks, modeled after a similar anti-government mobilisation in Spain.

The demonstrations were initially peaceful, with only sporadic scuffles breaking out at barricades put up by police around parliament.

But the violence escalated when scores of hooded youths began throwing stones, bottles and firebombs at riot police, who responded with heavy discharges of tear gas.

The semi-state Athens News Agency said at least nine people, including a prominent television journalist, had been injured, and the police department said five of its own officers were also hurt, some seriously.

"We have two-three officers who are in a serious condition," police spokesman Thanassis Kokkalakis told private Skai TV.

"One of them was hit in the ear by a flare and another has lost a number of fingers on his hand," he said, adding that two people had been arrested.

The protest was designed to turn up the heat on the government on a day when public transport and key services were paralysed by the general strike.

Papandreou had earlier held an emergency meeting with President Carolos Papoulias, the day after the government's parliamentary majority was reduced to just five seats by a lawmaker's defection.

Lawmakers are debating a new austerity package worth over 28 billion euros ($40 billion), a condition demanded by Greece's creditors in return for a badly-needed new aid bailout.

The plan is to be voted on by the end of the month.

"A national effort is demanded, we face critical decisions," Papandreou said ahead of the meeting in televised statements.

"We will take the necessary decisions to take the country out of the crisis ... (but) everyone must accept their responsibilities," the premier said, calling on opposition parties to support the austerity drive.

A similar economic bailout in Portugal prompted the collapse of a Socialist government followed by snap elections that were won by a centre-right party.

Reports said the government may seek to force opposition deputies to shoulder some responsibility for the reform package by setting a minimum majority for its passage of 180 votes in the 300-seat body, 25 more than the ruling party holds.

But the main opposition conservatives say they will vote against the measures.

Greece has warned it will be unable to pay next month's bills without a 12-billion-euro loan installment from the EU and the IMF, part of a broader 110-billion-euro bailout package agreed last year.

But the creditors have warned that no more aid will be forthcoming without firm reform commitments from Athens.

Many Greeks are angry that additional sacrifices are demanded after billions of euros in spending cuts and tax hikes last year.

Eurozone finance ministers failed to reach accord at talks on Tuesday on a second bailout package to avert a possible Greek default.