ATHENS, Greece (AP) -- The Greek government said Monday it will use emergency powers to prevent protesting teachers from disrupting university entrance exams this month. Civil servants' unions retaliated by calling a 24-hour strike for Tuesday.

It is the third time this year that the conservative-led coalition government has used the emergency civil mobilization order -- a measure normally reserved for natural disasters and other times of national crisis -- to end a labor dispute in the crisis-hit country.

More than 2,000 teachers -- some dressed in army fatigues to mock the order -- and left-wing unionists held two separate, peaceful protests in central Athens late Monday.

Tuesday's nationwide strike is expected to close schools and disrupt public services. Civil servants' union officials told the AP on Monday that the union was also planning a work stoppage Thursday and a series of public protests.

Government spokesman Simos Kedikoglou said the government decided to use the civil mobilization order against the teachers under its "moral obligation" to safeguard the May 17-31 exams for school leavers and university candidates.

"We are dealing with a threat to the national interest," Kedikoglou told private Antenna television Monday. "We cannot accept a situation that allows 100,000 children and their families to be held to ransom."

Teachers' unions had planned to start rolling strikes on May 17 to protest longer working hours and involuntary staff transfers.

The Greek Federation of Secondary Education State School Teachers, OLME, called the decision "extreme and undemocratic" and said it would challenge the order in court. The union, which is organizing a protest in Athens and other cities later on Monday, said it would review its strike plans on Tuesday.

Prime Minister Antonis Samaras signed the order, which took effect Monday, after taking similar action earlier this year to end strikes by workers at ferries services and Athens' subway train system.

"Strikes during the university entrance exams are unacceptable," Samaras said Monday. "Our society considers them repulsive. ... Union mentalities cannot cause severe social disruption just because teachers have been asked to work an extra two hours a week."

Pay and staff cuts have become routine in the debt-strapped country, part of the budget austerity measures demanded by international rescue lenders who have kept Greece from defaulting on its debts since it lost bond market access in 2010.

The draconian cuts have seen Greece's minimum wage slashed and unemployment rise to 27 percent.

"We don't think this kind of martial law should be invoked when dealing with industrial relations," said Tom Jenkins, a senior advisor to the European Trade Union Confederation, based in Brussels, Belgium.

Greece's left-wing opposition leader Alexis Tsipras said the cuts were prompting the government to take increasingly autocratic measures.

"I think the first casualty of the bailout agreement is democracy," Tsipras told private Sto Kokkino radio. "Continual states of emergency do not allow for law and order but their collapse. This government is dangerous for democracy."

Eurozone finance ministers will meet in Brussels later Monday and are expected to approve the payout of new bailout installments to Greece after more austerity measures were approved last month, including provisions for the mass dismissal of civil servants.

"The government has made great efforts to fulfill the conditions to secure the disbursements," said Martin Koehring, an analyst at The Economist Intelligence Unit.

He said the Greek government hopes the loan payments will be made by the end of this week, as it has 5.6 billion euros ($7.27 billion) in bonds maturing next on May 20.

Even if the payouts were not agreed upon on Monday, Greece would not face default next week, Koehring noted.

"We expect that the government would be able to cover the financing gap by issuing Treasury bills which are, however, costly," he said.