The Serbian Education Ministry evacuated Wednesday over 100 schools in the capital Belgrade after receiving bomb threats amid concerns after two mass shootings killed 18 earlier this month.

The ministry said 78 elementary schools and 37 high schools in Belgrade received warnings by email early Wednesday that explosive devices had allegedly been planted.

Classes were postponed and students evacuated as police checked the buildings.

There have been no reports that bombs were found in any of the schools, and police are yet to issue their report.

Similar multiple threats have been sent to school addresses in the past, in Serbia and other countries in the region, and have proven false each time.

However, the threats could further heighten security fears after the May 3 and May 4 shootings that left 10 people dead and 21 injured.

Authorities already have deployed police in schools and launched a gun crackdown.

The first shooting happened at an elementary school in central Belgrade when a 13-year-old boy took his father's gun and opened fire.

A day later, a 20-year-old shot randomly at people with an automatic weapon in two villages south of Belgrade, killing eight.

The shootings stunned Serbia and sparked calls for action. Tens of thousands marched in two protests against violence after the shootings, and more protests are planned on Friday.

Opposition parties are demanding that ministers of interior and intelligence chief resign and that two pro-government networks be stripped of nationwide broadcast licenses because of airing violent content and hosting war criminals and crime figures on their program.

Opponents also have accused populist President Aleksandar Vucic of hate speech against critics and a crackdown on democratic freedoms, which they say fuel tensions and divisions in the troubled Balkan nation.

Vucic has denied this. He has called his own rally for May 26 while suggesting a snap election could be held by September.