Scuffles broke out at barricades set up by the police, who made sporadic use of tear gas to push the demonstrators away.[break]
Hooded youths threw stones and firebombs at the police but were also opposed by participants in the mass protest called by a group that has occupied central Syntagma Square for weeks.
Police said 20,000 people had responded to the call but media estimates raised the turnout to 40,000.
Another 20,000 demonstrated in the main northern city of Thessaloniki, authorities said.
Lawmakers inside the building were debating a new austerity package worth more than 28 billion euros ($40 billion), a condition demanded by Greece´s creditors in return for a badly-needed new aid bailout.
Prime Minister George Papandreou began an emergency meeting with the Greek head of state, President Carolos Papoulias, after a government deputy defected on Tuesday, reducing his Pasok party´s majority to five seats.
Another member of Pasok also recently indicated that he would vote against the government´s plan, raising the spectre of the reforms being rejected.
"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," he said, calling on opposition parties to support the austerity drive.
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 Greece´s right-leaning main opposition party insists it will vote against the measures.
Greece has warned it will be unable to pay next month´s bills without a 12-billion-euro loan instalment 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.
Dozens of police vans were parked in front of parliament Wednesday to allow the deputies unhindered access and keep at bay the crowd of protestors.
The square outside was awash with Greek national and Spanish flags and banners reading "Resist" and the battle cry from the Spanish civil war, "No pasaran" (They shall not pass), reflecting a similar protest movement in Madrid.
Rallies called by trade unions were due to begin later.
The general strike, the third this year against austerity, disrupted public transport on land and sea and forced offices to close.
"We will not allow those who plundered the country to also seize public assets and destroy the country," Stathis Anestis, a senior member of Greece´s main union GSEE, said in a statement.
Tension has been rising in Greece in the past few days with protestors voicing their discontent over plans for a new wave of spending cuts and tax hikes.
Many Greeks are angry that additional sacrifices are demanded after billions of euros in spending cuts and tax hikes last year.
The drive to sell off 50 billion euros in state assets to reduce a climbing debt of 350 billion euros has proved particularly galling.
"The politicians lied to us," said Maria Chira, a 30-year-old mother of two.
"Greece is in danger, this is the most important mobilisation since the seventies," she said, referring to a student uprising that helped bring about the collapse of a military dictatorship in 1974.
"We no longer have our fates in our hands," added 70-year-old Machi Spyridaki.
"I am here to defend our country´s pride and the future of my children and grandchildren who are destined to a life of unemployment and poverty wages," she told AFP.
Eurozone finance ministers failed to reach accord at talks on Tuesday on a second bailout package to avert a possible Greek default.
Opinion polls show most Greeks have lost confidence in their country´s government and a political and judicial system that has conspicuously failed to root out endemic corruption.
Related story
Movies that capture the essence of Greek tragedies
