Greek police fired tear gas as teachers began a strike against public sector layoffs and forced transfers.
Police moved in to disperse school guards trying to enter the Administrative Reforms Ministry in central Athens.
School guards patrol school premises and oversee road crossings.
Teachers started the open-ended strike against moves to place public sector workers in a so-called mobility scheme for transfers to schools elsewhere in the country or possible dismissal.
The school guards are covered by the scheme along with administrative staff working for Greece's largest social security fund, the IKA, who also intend to join the strike.
Athens needs to deliver on the scheme to satisfy demands by the country's international creditors for further bailout funding.
The plan, part of moves to reform and shrink the public sector, has met fierce resistance from labour unions.
Secondary teachers' union OLME began the five-day rolling strikes starting today, saying firings or job transfers will add to the pain felt by austerity-hit households.
Primary school teachers will also hold a three-hour work stoppage.
OLME is also expected to join a 48-hour strike on 18-19 September called by Greece's largest public sector union ADEDY.
State hospital doctors will start a three-day strike tomorrow.
Lawyers will also walk off the job for two days, opposed to reforms to open up their profession.
Greece must place 25,000 workers in the mobility scheme by the end of the year, where they will be evaluated and transferred to other jobs or dismissed.
It must also meet a target of 15,000 mandatory public sector exits in 2013-14.
Firing public sector workers is a sensitive issue in the crisis-hit country, where unemployment has hit nearly 28% and poverty levels have soared during a six-year recession.