ATHENS—A largely peaceful protest Wednesday by tens of thousands of Greeks against new government austerity measures was marred by violence in central Athens late in the day, when hundreds of youths wearing ski masks hurled water bottles, firecrackers and other objects at police, who responded with tear gas and pepper spray.

Still, the one-day protest was restrained by the standards of recent Greek actions. It came a year after a mass antigovernment demonstration turned deadly as unknown assailants firebombed a bank building,...