As MPs debated a new austerity package in Parliament on Tuesday, thousands protested the new measures in rallies in central Athens that were marred by isolated outbreaks of violence.
Meanwhile most transport and public services ground to a halt as workers launched a 48-hour general strike, the first two-day walkout since the restoration of democracy in 1974.
The Athens metro was the only form of public transport operating, and it will continue to run on Wednesday to allow demonstrators to travel to the center.
Flights are to be disrupted again Wednesday as air-traffic controllers stage more stoppages and protesting dock workers have threatened to block the port of Piraeus again after stopping travelers from boarding ferries yesterday.
The demonstrations - some organized by labor unions, others by the self-proclaimed ?Indignant? protesters - were planned to precede two critical votes in Parliament - one on Wednesday on the new austerity package, dubbed the midterm economic program, and the second on Thursday on the implementation bill for this program.
The rallies started peacefully but were marred in the early afternoon when groups of hooded youths started hurling stones and firebombs at riot police officers who were out in force. Police responded by firing tear gas to disperse the crowds.
A police spokeswoman said 23 people were detained, with five of them later arrested, and 21 police officers were hurt but none seriously. The National First Aid Center (EKAB) said six protesters were injured while local media reports put the number much higher.