Greek Prime Minister Lucas Papademos insisted he would press ahead with spending cuts despite thousands of unionists in major cities taking to the street to protest austerity plans. Police put the demonstrators at 17,000 in Athens and 6,000 in the northern city of Thessaloniki as the sixth general strike this year shut down public services and crippled train and ferry services. The last general strike in October had attracted over 120,000 people nationwide. Flights were not affected and the Athens stock exchange was open, as was most of the capital\'s metro system. In central Athens, riot police were out in force around parliament as union posters of \"No Austerity Budget!\" hung from lamp-posts and a crowd of about 3,000 gathered in Syntagma square, scene of violent clashes in the past. Police said there were no arrests while state TV reported one incident -- a group of some 150 youths throwing firebombs and setting fire to a car in the left-wing stronghold of Exarchia in the centre of Athens. Papademos, in power since only November 11, wrote to the EU and International Monetary Fund that his government would implement the tough measures agreed in October in a second bailout for debt-stricken Greece after a May 2010 rescue. \"The government will take all measures necessary in order to implement the (October accord) ... and achieve the objectives of the economic programme, which is crucial for improving the long-term welfare of the Greek people,\" he said. The IMF signalled that its next loan installment to Greece could be released next week. A spokesman, Gerry Rice, said at a news conference that the Fund\'s executive board would meet Monday to discuss the aid to the struggling eurozone country. Greece needs the 8.0-billion-euro ($10.8 billion) EU-IMF loan installment by December 15 to pay its bills. The loan had been frozen in August pending confirmation that Athens had met its conditions. The IMF loan installment is for 2.2 billion euros. Eurozone governments on Tuesday agreed to release their 5.8-billion-euro share as Greece teeters toward default. Union leaders said people were fed up with spending cuts and higher taxes and warned they could seek common cause with groups in other countries. Kostas Tsikrikas, head of the ADEDY civil servants union, said \"workers continue their struggle against this unfair, barbarous policy ... We are going to coordinate our action with unions in other southern (eurozone) countries.\" Tsikrikas conceded there were fewer protestors than at the last general strike October 19 which attracted around 120,000 but added that \"the problems are going to get worse, we have had enough and we can\'t go on like this.\" The protests follow similar strikes across Europe against spending cuts and tax increases introduced by governments fearful of getting sucked into the eurozone debt quagmire. Yiannis Panagopoulos, chairman of private-sector union GSEE which organised the strike with ADEDY, told AFP Wednesday that there \"must be no illusions, austerity will continue in 2012 and so will our mobilisation because insecurity and the threat of unemployment persist.\" Successive austerity packages have become tougher over the past two years as Athens has cut spending and hiked taxes in an effort to balance the public accounts, inflicting pain on many. Some marchers wanted an end to widespread tax evasion, blamed by many for the parlous state of the public finances with up to 60 billion euros unpaid, according to EU figures. Others called for measures to boost growth -- the Greek economy is mired in a deep recession -- rather than more austerity. Rosina Kolonia, a retiree, said the government -- a technocratic administration formed only to implement the tough reforms tied to Greece\'s second bailout -- had no right to push the changes through. \"I am here because the government was not elected ... it can\'t commit itself nor commit the Greeks since it was not elected,\" Kolonia said, noting that her pension has been cut by 25 percent since 2010. \"The spreading of the (eurozone debt) crisis shows that it is not the fault of the Greeks, it is the financial system that caused it,\" she added. Papademos, a former European Central Bank vice president, heads a government which includes members of the opposition conservative and far-right nationalist parties who previously attacked the reforms agreed with the EU and IMF. They have pledged, however, to support the measures which go to a vote -- and expected passage -- in parliament just before another eurozone debt crisis summit in Brussels on December 8-9. GSEE and ADEDY, which claim a million members, have held more than a dozen general strikes against austerity plans in the past two years as Greece, saddled with a 350-billion-euro debt mountain, struggles to keep its head above water.