WATCH: Greek police fired tear gas to disperse a group of protesters demonstrating against planned austerity measures outside the parliament building in central Athens. (Video by Reuters)
Trade unions in Greece have launched a 48-hour general strike just hours after Prime Minister George Papandreou urged parliament to back tough austerity measures.
Tens of thousands of protesters took to the streets of several Greek cities, including two marches on the parliament in Athens.
In the capital, police fired teargas to disperse small groups of protesters throwing sticks and bottles.
Police said 18 people were detained, with five of them later arrested. At least four police officers were injured.
Some 5,000 police had been deployed in anticipation of possible confrontations.
Legislative approval of the austerity measures could free up some 12 billion euros in international loans to Greece. The government says that unless it gets the loans, the country could run out of money and default on its sovereign debt.
French banks on June 27 gave Athens more time to make payments on the country's crushing debt.
Unions say that the austerity measures would hit hard minimum and low wage earners in the country, where unemployment is already at 16 percent.
About 20,000 protesters marched to parliament in Athens in two separate demonstrations before police intervened against groups hurling objects at them.
Another 7,000 marched in the country's second-largest city of Thessaloniki in the north.
The strike was disrupting most public services, with doctors, ambulance drivers, journalists, and even actors at a state-funded theater joining the protest.
Papandreou told parliament the austerity package is the only way to get Greece back on its feet.
Passage would trigger the release of sizable loans to Greece from the European Union and International Monetary Fund (IMF).
compiled from agency and media reports