BY ALKMAN GRANITSAS, ANDREA THOMAS AND NEKTARIA STAMOULI

ATHENS—Clashes broke out in central Athens Friday as Greece’s major unions launched a 48-hour nationwide strike to protest new austerity measures demanded by the country’s creditors, adding to social tension in a country now in its fifth year of economic recession.

Hundreds of hooded youths in the main Syntagma Square in Athens attacked riot police with gasoline bombs and other projectiles, smashing objects in and around the square.

Police fired tear gas and staged running charges to scatter the youths. At least one person was detained.