Some 2,000 cab owners who oppose deregulation demonstrated in the city centre, brandishing Greek flags and shouting slogans against the government.
The protesters passed in front of the finance ministry, where civil servants occupying the building also waved Greek flags and shouted anti-austerity slogans.
Public sector staff have regularly taken over state buildings in recent days to protest against a new round of layoffs and pay cuts adopted to trim the country's bulging deficit.
Customs inspectors also began a ten-day strike on Friday, raising the prospect of fuel shortages.
A full-blown 48-hour general strike will be held on October 19 and 20.
The government urged unions to show common sense, and threatened to dock strikers' pay.
"Some are playing with fire, with the future and the perspective of the Greek people because they do not understand, or worse, they do understand and are sacrificing a common purpose for union or partisan interests," Evangelos Venizelos told parliament.
As debate began on a new set of austerity measures needed in return for rescue loans from the European Union and the International Monetary Fund, the minister called for "national unity (in) these crucial and uncertain moments faced not only by Greece but by all of Europe and the United States."
"You can't occupy the general accounting office, the Social Security IT centre or the national printers. All this threatens democratic legitimacy and democracy must defend itself," Venizelos pleaded in response to a series of civil servant sit-ins that have hampered the normal running of government.
For the second day running, a general transport strike brought Athens to a standstill with buses, trams and the underground blocked and taxis also on strike against a planned deregulation of their sector.
"The government seeks to nationalise 50,000 families who paid for their cab licences with sweat and sacrifices," the head of the taxi owners union Thymios Lyberopoulos told Flash Radio.
"They will pass over our dead bodies," he warned.
Greece's two main unions said Thursday they would stage a two-day general strike next week to protest at the new government austerity cuts intended to resolved a debt crisis that has shaken the eurozone.
The strike is timed to coincide with a vote in parliament on the new batch of austerity measures as part of reforms mandated by the EU, the IMF and the European Central Bank, which last year earmarked a 110 billion euro ($151 billion) loan for Greece.
Speaking in Brussels on Thursday during talks with European Council chief Herman Van Rompuy and eurozone head Jean-Claude Juncker, Prime Minister George Papandreou said Greeks were determined to make "painful" changes to reform.
"We want to change in Greece. And however painful, we are committed to make these changes in the best interests of the Greek people," Papandreou said.
More than 1,000 public workers demonstrated in Athens on Thursday to protest against the cuts which they say are driving salaries to sub-poverty levels.
Other unionists blocked the interior ministry, forcing Interior Minister Haris Kastanidis to postpone a news conference on tackling a 10-day protest by garbage collectors that has seen thousands of tonnes of refuse accumulate in Athens and Thessaloniki.
The government has threatened to bring in private contractors to clean up the garbage and to leave strikers without pay.
"We obviously need to defend public health, and this means bringing in private contractors if need be," Kastanidis told Flash Radio on Friday.
He argued that the cost to the state would be a third of what it currently pays to public garbage collectors.