Greek legislators have approved more austerity measures needed to avert a debt default next month, but in doing so have triggered a second day of riots in the central square in Athens that left dozens injured and the capital covered with tear gas.

By Wednesday night, police said 49 officers had been injured, one seriously when he was hit in the face by a chunk of marble. Forty-three protesters were detained, with 17 of them arrested. Emergency services said they had treated 99 protesters and passers-by for injuries.

Head medical volunteer Flegas Stagos said more seriously wounded protesters were put onto the metro with volunteers so they could receive treatment away from the tear gas.

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At one point, protesters set a fire in a post office on the ground floor of the building housing the Finance Ministry, but the flames were soon put out.

The bill passed 155-138, with the support of all but one government member.

While hooded young men, described by police as anarchists, threw firebombs and rocks and were met with tear gas, several hundred protesters tried to hold a peaceful demonstration in the centre of the square.

CBC journalist Susan Ormiston reported witnessing an "unbelievably violent" day. She saw 50 police on motorcycles charge into the square, trying to push rioters back and end two days of disturbances.

Some demonstrators told Ormiston they felt ashamed that the parliamentarians did not come outside to hear what they had to say before the vote.

Most of the anti-government protesters who marched to the square stayed clear of the fighting, but they vented their anger at the political establishment with chants and insults.

A general strike continued for a second day, grounding planes, docking ferries and stranding tourists during the busy summer season.

Britain faces austerity protests Thursday

Britain is also facing austerity protests beginning Thursday, when 750,000 public-sector workers — from teachers to driving examiners to customs officials — walk out for the day, closing thousands of schools and leaving travellers to face long lines at airport immigration offices.

The austerity bill provides for tax increases, spending cuts and privatization of facilities such as ports and airports due to raise $40 billion, but which has provoked days of rioting in the streets of Athens.

Greek legislators must next turn their focus to an additional bill to be voted on Thursday that details how the austerity will be implemented.

In a joint statement, the heads of the EU commission and council, Jose Manuel Barroso and Herman Van Rompuy, said Greece had taken "a vital step back — from the very grave scenario of default."

A firefighter tries to put out a fire at the central post office while a petrol bomb, thrown by protesters, explodes behind him during clashes in central Athens Wednesday. (Petros Giannakouris/Associated Press)

Both measures must pass if the European Union and International Monetary Fund are to release the next €12 billion slice of the country's €110 billion ($155 billion) bailout fund — and prevent a default that could drag down European banks and shake the European and world economy.

Even if Greece gets more bailout funds, many economists think the country will end up defaulting on its debts in some form or another. Implementing the measures is not going to be made any easier if the widespread opposition continues.

"While approval of the package is an essential first step toward Greece getting the much-needed funding it needs from the EU/IMF to meet its upcoming financial obligations, they are not out of the water just yet," said Carl Campus, an analyst at BMO Capital Markets.

Prime Minister George Papandreou's Socialists have a slim majority of five seats in the 300-member parliament, and he has faced an internal party revolt over the new punishing four-year program of spending cuts and tax hikes on even those on minimum wages.

Papandreou needed 151 votes to pass the bill, and by the late afternoon local time in Greece, it became clear the government had enough votes to win it. Many lawmakers apparently abstained from participating in the controversial vote.

Greece has said it has funds only until mid-July, after which it will be unable to pay salaries and pensions, or service its debts, without the next bailout instalment. The country is also in talks for additional help in the form of a second bailout, which the prime minister has said will be roughly the size of the first.

Dissatisfaction over the new measures, and the realization that harsh cuts and tax hikes imposed over the past year have not worked as expected, has prompted sharp criticism even from within Papandreou's party and led to a political crisis earlier this month that saw the government nearly collapse.

With files from The Associated Press