Greek demonstrators clashed with riot police Tuesday in Athens during the fifth major strike this year, called to protest government austerity measures.

An estimated 10,000 people took part in Tuesday's protest marches, which shut down transportation services, left hospitals operating with emergency staff and pulled news broadcasts off the air.  Some protesters threw projectiles at police, who responded with tear gas.

Greece's two main private and public sector labor unions - GSEE and ADEDY - organized the nationwide walkout.  The unions fiercely oppose the government's moves to raise taxes, freeze civil service pay and cut pensions in a push to decrease the country's huge debt.

Tuesday's protests are small compared to demonstrations in Athens last month.  A rally May 5 attracted tens of thousands and ended with the deaths of three bank employees inside a building that was set on fire.

Greek officials say the austerity measures being considered are necessary to avoid bankruptcy. Greece agreed to the steps to secure a massive emergency loan from the European Union and International Monetary Fund.

Members of a communist-affiliated union (PAME) also demonstrated Tuesday in Greece's main port of Piraeus, where they tried to block tourists and locals from boarding ferries at the port.

Some information for this report was provided by AP, AFP and Reuters.