In one of the biggest demonstrations in the capital in recent years, as many as 200,000 marched on the Greek parliament, according to unions in the public and private sector, which called the strike to oppose new wage and pension cuts – the price of further rescue funds from international lenders.

Clashes broke out between riot police and hooded youths hurling rocks and petrol bombs at the finance ministry. The protesters, many shouting: "We can take no more. Out with the EU and IMF," and said to be part of the crisis-hit country's vibrant "anti-establishment" movement, then set light to rubbish cans and bus stops, sending plumes of acrid smoke above the capital. TV footage showed demonstrators running for cover in Syntagma Square, seat of the Greek parliament, as noxious fumes filled the air. More than 100 people were detained.

With tensions running high over the new measures – part of a mammoth €11.9bn austerity package that is set to save the state more than 5% of GDP over the course of the next two years – unions are warning that the conservative-led government should take stock or worse could come.

In a departure from other mass protests, members of the police force, army, navy and judicial system joined public and private sector employees on the streets. One police officer, who preferred not to give his name, said the Greek state "should feel deep shame" at imposing cuts on the very people whose protection it sought.

"This is a warning to the government not to pass the measures," said Ilias Iliopoulos at ADEDY, the union of civil servants, insisting that around 350,000 Greeks took part in protest marches nationwide (police put the number in Athens at around 70,000). "Today was a huge success as witnessed by all those in the armed forces and police who also participated because they, too, will be affected by these cuts. The government must know that if it wants to push us further into a corner, we will react."

Echoing a view held by many Greeks, Penelope Angelou, an unemployed mother, said passing the measures would be tantamount to a "parliamentary coup".

"These parties were given our vote back in June because they promised to re-negotiate the terms of the loan agreement," she said, referring to the onerous conditions of the bailout accord Athens signed with its "troika" of creditors — the EU, ECB and IMF – earlier this year. "We are all tired," she said. "This is the third year of non-stop cuts and tax increases which have made us poor and divided us as a society. And they have not solved our problem. The recession is going from bad to worse."

Since the outbreak of Europe's debt crisis in Athens in late 2009, ordinary Greeks, worst hit by repeated rounds of austerity, have seen their purchasing power drop by as much as 50% as poverty and joblessness has reached record levels.

After a heated summer of tortuous wrangling with lenders, the alliance led by the prime minister, Antonis Samaras, is expected to finally give the package the go-ahead on Thursday. Time is of the essence, say officials, if the Greek economy is to receive a bumper rescue loan of €31.5bn, put on hold by the troika since July. The injection is now vital to "warming up" the cash-starved real economy. Creditors have made clear that without the controversial measures, there can be no money – a scenario that would see Greece defaulting on its debt, being forced to declare bankruptcy and leaving the eurozone.

"The cuts have to happen because we are at war, an economic war," the country's defence minister, Panos Panagiotopoulos, said this week.

Once endorsed, the package will be sent to the parliament for ratification, probably next week. But the government's highwire act of trying to placate lenders while ensuring that the nation is not pushed over the edge will not be easy. Polls have shown the vast majority of Greeks see the measures as deeply unfair and antisocial. Highlighting fears that the recession-inducing policies have pushed Greece into an economic death spiral, the ratings agency Fitch declared on Wednesday that far from being reduced, Athens's debt mountain was growing, with the country's debt-to-GDP ratio set to increase from its current 164.9% to 180.2% in 2014.

"Once the Greek people learn exactly what the measures are there will be uproar," Iliopoulos, the trade unionist, told the Guardian. "Parliament will see mass protests. And it won't be nice."