A public prosecutor has ordered an investigation into alleged police brutality against Greeks protesting against austerity measures amid an outcry over the excessive use of force and teargas to control crowds in Athens this week.

As municipal employees worked furiously to clean up the capital in the wake of street battles that left its central square resembling a war zone, the socialist government faced growing criticism of the controversial methods employed by riot police to disperse demonstrators.

"What took place in the centre of Athens these past few days is a complete violation of democratic law," said Alexis Tsipras, leader of the leftist Syriza party, emerging from the supreme court where he filed a suit against the Greek police.

Fierce fighting erupted outside the Greek parliament on Wednesday as MPs inside voted on the hard-hitting policies demanded by the EU and IMF in exchange for the debt-choked country receiving further aid.

The release of video footage depicting police beating protesters, and in one instance seemingly colluding with rock-throwing extremists before firing off rounds of tear gas, has shocked Greeks.

An estimated 500 civilians were injured in the violence, with many suffering serious respiratory problems after police, firing volleys of teargas, released dangerously high levels of toxic chemicals into the air. The widespread use of the gas was crticised by Amnesty International.

Doctors who set up an emergency hospital said they were attacked by police as they tried to administer aid. In another incident, police were also seen appearing to throw broken bits of masonry at protesters when tear gas supplies dried up.

"I have a complete dossier with two discs of audio visual material and 38 different videos … which eloquently portray scenes of raw and unprovoked violence by police against peaceful, unarmed citizens," said Tsipras. "[The videos also contain] scenes that show evidence of the collaboration between police forces and the suspicious hood-wearing thugs." These "thugs" have often been blamed for provoking violence at demonstrations.

Put on the defensive the government apologised on Friday for the overzealous use of force.

But just days after Athens narrowly averted default – and by extension a Europe-wide crisis – by approving the €28bn package of spending cuts and tax increases, there is a growing sense that the feat was accomplished at the expense of ordinary Greeks.

The policies have been fiercely opposed by lower-income groups, especially civil servants already hit by a year of deep austerity.

Ahead of the vote, demonstrators, including thousands of pensioners and middle-aged men and women, had vowed to blockade the parliament in a bid to physically stop lawmakers casting ballots.

"The government was clearly determined to do anything to pass the measures and please those who govern us in Brussels," said Iason Nika, a student who has joined the Movement of Indignant Citizens encamped in Syntagma Square outside the 300-member parliament. "Police followed an organised plan to clear the square with teargas and sheer brutality. It was junta tactics all over again almost 40 years after the collapse of military dictatorship."

Court prosecutor Eleni Raikou ordered the investigation after a complaint by a union leader. Yesterday, the Greek premier George Papandreou called Europe's decision to give Athens a vital fifth tranche of aid – needed to avoid bankruptcy – a "vote of confidence" in Greece.

"We have taken difficult decisions in trying times," he said in an address to leftist leaders attending a Socialist International meeting in Athens. "These were patriotic decisions to save our country, socially necessary decisions and painful decisions … decisions to give us time, opportunity and the hope for deep and real change in Greece."