Riot police have scuffled with striking school guards in central Athens as unions gear up for a series of public sector strikes over job cuts.
At least two demonstrators were taken to hospital suffering from breathing problems after police used pepper spray in an attempt to move protesters away from the Administrative Reform Ministry.
Separately, thousands of teachers and other civil servants held an afternoon demonstration to parliament to demand a repeal of government plans to axe thousands of public sector jobs. In the northern city of Thessaloniki, about 10,000 protesters held a similar march. The country's main civil servants' union has called a two-day public sector strike for Wednesday and Thursday.
Under government plans, 12,500 public sector employees are to be suspended as part of efforts to trim Greece's bloated public sector and to meet conditions of the country's international bailout.
Greece has been depending on rescue loans from the International Monetary Fund and other eurozone countries since May 2010. In return, it has imposed repeated rounds of austerity measures, slashing salaries and pensions and introducing several rounds of tax increases.
While the budget figures are showing some signs of improvement, with the deficit narrowing, the measures have also left the economy hammered by a deep recession, now in its sixth year. Unemployment has risen to above 27%, and is nearly 60% for those under 25.