Unions representing half the 5-million-strong workforce also launched a nationwide strike, shutting government offices, ports, schools and reducing hospitals to skeleton staff.RELATEDNetanyahu, Greece’s new lobbyistPrime Minister George Papandreou must push through a new five-year campaign of tax hikes, spending cuts and sell-offs of state property to continue receiving aid from the European Union and International Monetary Fund and avoid default.Thousands of activists and unionists converged on the central Syntagma square on the parliament's front steps to try to stop lawmakers from entering to debate the bill in committee that they hope to pass by the end of the month.Stun grenades boomed around the square and plumes of smoke rose from burning garbage bins as police fired tear gas and fought running skirmishes with scores of youths who fought back with rocks and long clubs, Reuters witnesses reported.One group hurled petrol bombs and clashed with police at buildings housing the Finance Ministry, also on the square. Reuters witnesses saw flames in front of an entrance to the main building and a similar clash a few buildings down.The vast majority of the diverse crowd -- which included union workers, political party members, pensioners, and a wide array of Greeks upset at the new austerity measures -- only shouted at the parliament building and remained peaceful.Police said seven protesters and two officers were slightly injured and they had detained 40 people. They said the crowd numbered around 30,000 but they often underestimate numbers.The new austerity package foresees 6.5 billion euros ($9.4 billion) in tax hikes and spending cuts this year, doubling measures agreed with bailout lenders that have pushed unemployment to a record 16.2 percent and extended a deep recession into its third year.