Both groups embarked on a two-day walkout ahead of industrial action called by civil servants, doctors and Communist-backed workers on February 10 and a general strike called by Greece's main umbrella union on February 24.
"We have already made sacrifices and will accept no more cuts," the chairman of the customs officers' union Argyris Sakellaropoulos told Flash Radio.
The unions are on the warpath over a government austerity programme that they say has progressively become harsher under pressure from the European Union and market speculation that has hurt Greek finances and rattled the euro.
"I deeply regret that the government has bowed to the wishes of the markets," the head of the General Confederation of Greek Workers (GSEE), Yiannis Panagopoulos, said on Wednesday.
Greece's largest union with around 600,000 members, GSEE is expected to approve Panagopoulos' call for a February 24 general strike later Thursday.
GSEE has also walked out of the official debate on how to reform the country's cash-strapped pensions system that is a key part of the government's economy rescue agenda.
The country's recession-hit economy has already been disrupted by a protest by farmers, who have blocked key highways and shut down border crossings on a near-daily basis for three straight weeks.
Greek Prime Minister George Papandreou on Tuesday ordered a public salary freeze, a higher retirement age and a hike in petrol prices, supplementing a crisis plan unveiled last month to reassure the international financial community.
Greece's debt stands at more than 294 billion euros ($412 billion), its 12.7-percent deficit is well beyond EU limits of three percent of output for eurozone members and it suffered a triple downgrade of its sovereign debt in December.
The government hopes to economise over a billion euros ($1.4 billion) from benefit cuts and operational cost reductions in the public sector.
Overall, Athens aims to save over 10.3 billion euros ($14.3 billion) this year with improved tax collection, state cost cuts and reduced arms spending to bring the public deficit to 2.8 percent of output by 2012.
The programme also seeks to stabilise Greece's debt burden -- one of the highest in the eurozone -- and reduce it to 117.7 percent of GDP by 2012.
It forecasts an economic contraction of 0.3 percent in 2010, and a return to growth of 1.5 percent in 2011 and 1.9 percent in 2012.
But Papandreou, whose Socialists came to power in October, has been accused of breaking pre-election pledges and undermining social dialogue.
"The adoption of these measures raises suspicion that the government could carry out additional upsets, eliminating retirement rights and even slash pensions, when it had categorically denied that it would do so," leftist Eleftherotypia daily said on Thursday.
"Draining the people of money is not enough to cure the economy, when the money will be spent on past and future heavy loans," said right-wing Apogevmatini daily.
The finance ministry has already indicated that it will take further measures if it deems them necessary.
In a badly-needed breather, the European Union on Wednesday approved Greek efforts to tame the debt crisis but placed Athens under surveillance measures.
The decision to place Greece under a permanent system of surveillance while it monitors the reform plans was a first for the EU, which also rapped Athens over faulty budgetary data.
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