Demonstrators in Athens clash with riot police during a nationwide general strike in protest at austerity measures Reuters

Violence erupted in the heart of Athens as mass protests against further austerity measures in crisis-hit Greece escalated on Wednesday.

Police fired tear gas at crowds throwing rocks and petrol bombs. The exchange disrupted an otherwise peaceful march through the capital by up to 200,000 demonstrators participating in a general strike, the first big confrontation with Greece's three-month-old coalition government.

Industrial action, hailed as a triumph by unions, brought the country almost to a standstill, grounding flights, disrupting transport and shutting public services including tourist sites.

The governing coalition, under immense pressure to pass yet more cuts by international creditors keeping the moribund Greek economy afloat, had ordered bulletproof barriers to be erected around the parliament but had hoped the protests would end peacefully.

As smoke rose over parts of the city and protesters donned gas masks, it was clear those hopes had been dashed. About 3,000 officers – twice the number usually deployed – had been standing guard in central Athens as authorities braced for rioting.

"This is a warning to the government not to pass the measures," said Ilias Iliopoulos at the ADEDY, the union of civil servants, insisting that about 350,000 Greeks took part in protest marches. "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 wants to push us further into a corner, we will react."

Prime minister Antonis Samaras's conservative-led alliance is expected to decide on Thursday on budget cuts totalling €11.9bn (£9.46bn) – more than 5% of the country's GDP. Once endorsed, the controversial austerity package will be sent to parliament for ratification. "Once the Greek people learn exactly what the measures are, there will be uproar," Iliopoulos said. "There will be mass protests."

Ships stayed docked, shops pulled down shutters, and museums and monuments – including the Acropolis – closed to visitors. Air traffic controllers walked off the job for three hours and hospitals were operating with emergency staffing levels only.

Much of the union anger is directed at the additional spending cuts over the next two years that Greece has promised its "troika" of lenders – the European Central Bank, the European commission and the International Monetary Fund – in order to secure its next tranche of aid. The bulk of the cuts are expected from slashing wages, pensions and welfare benefits – heaping new misery on Greeks, who say repeated rounds of austerity have pushed them to the brink yet failed to transform the country for the better.

A poll by the MRB agency last week showed that more than 90% of Greeks believe the planned cuts are unfair and a burden on the poor, with the vast majority expecting more austerity in coming years.

With the country in its fifth year of recession, analysts said a strong public backlash could tear apart the weak coalition.

"What people want to tell Samaras is that they are hurt – and Samaras could use this to demand concessions from the troika," said MRB's polling director, Dimitris Mavros.

"The people are willing to give the government time, but on certain conditions like cracking down on tax evasion and securing a bailout extension. If the government succeeds in that, its life will also be extended."