Riot police in Athens have fired tear gas against masked youths during a march by tens of thousands of Greeks protesting austerity measures demanded by the new unity government. Police said 27,000 people in Athens and 15,000 in the second city of Thessalonika joined demonstrations marking a 1973 student uprising seen as a key moment in the restoration of democracy in Greece nearly four decades ago. 'We will throw all of them out,' promised a banner held aloft by students in Athens while another carried by anarchists read: 'In the face of tyranny, one must choose between chains and arms.' Thursday's march, the first test of the scale of public defiance against the coalition set up to deal with Greek's crippling debt crisis, was a sombre, largely peaceful affair although violence flared briefly during the afternoon. A group of protesters threw stones and two firebombs at police outside parliament, while a dozen masked youths set two large dustbins alight outside police headquarters, near the US embassy, police said. Riot officers wearing body armour and brandishing batons and shields responded with tear gas, protecting themselves with gas masks as the crowds ran away. At least 23 people were detained, police and an AFP reporter said. An empty guard box outside European Union offices was also set on fire during the demonstration, which dwarfed the 20,000 who gathered in Athens last year. Greece is slogging through a third year of recession exacerbated by wage cuts and tax hikes imposed by the previous socialist government of George Papandreou. These are set to continue under the new coalition administration set up last week under former European Central Bank deputy chief Lucas Papademos, as he seeks to implement reforms demanded by EU and International Monetary Fund creditors in exchange for a $US130 billion euros ($A129.14 billion) bailout. 'We have no money, no jobs, no future in this country,' said Sotiris Kirbas, a 52-year-old unemployed man who joined the protests in Athens. 'The crisis is not only about numbers, it's about people.' Police had deployed 7000 officers across the capital in anticipation of trouble. The annual march of November 17 commemorates a student uprising at the Athens Polytechnic in 1973 which was violently repressed. At least 44 people died but it helped topple a US-backed army dictatorship and brought back the republic. The bloodstained Greek flag that flew over the university that night is carried at the head of the demonstration each year. Papademos said the anniversary was 'an opportunity to emphasise that with determination and unity, we can achieve our national goals to resolve the crisis and guarantee the country's return to growth and jobs'. But the inclusion of far-right politicians in his new unity cabinet, the first time they have been in power since democracy was restored in 1974, added to the outrage of the protesters. 'Down with the government of socialists, conservatives and fascists,' a protester banner said. The socialist, conservative and far-right nationalist parties formed a coalition last week to save Greece from looming bankruptcy, and they must now approve a crucial eurozone debt bailout before holding early elections. 'We have an economic and political junta,' said Marita, a 23-year-old civil engineering student enrolled at the Polytechnic. 'We have a government that did not emerge from elections, nobody asked us. What is this, if not a junta?' she said. The new government was officially confirmed in parliament late on Wednesday, with 255 out of 300 MPs showing their support in a vote of confidence. But Greece's third largest party, the Communists, and the smaller leftist Syriza party have pledged to fight to bring down the government to prevent further belt-tightening after two years of austerity.