Serbian citizens have handed in nearly 6,000 unregistered weapons in the first three days of a month-long amnesty period that is part of an anti-gun campaign following two mass shootings last week, police said Thursday.

The police also received about 300,000 rounds of ammunition and about 470 explosive devices during the same period, the Serbian Interior Ministry said on Instagram.

The effort to rid Serbia of excess weapons began after 17 people were killed in two mass shootings last week and 21 were injured, many of them children. One of the shootings took place at a school for the first time in Serbia.

Authorities have told citizens to turn in unregistered weapons by June 8 or face prison terms. Other anti-gun measures include a ban on new gun licences, tighter controls on gun owners and shooting ranges, and tougher penalties for illegal gun possession.

The school shooter was a 13-year-old boy who used his father’s gun to open fire on his classmates at a primary school in central Belgrade last Wednesday, police said. A day later, a 20-year-old man opened fire with an automatic weapon in a rural area south of the capital city.

Serbia is estimated to be among the top countries in Europe when it comes to per capita gun ownership, which is partly remnants of the wars of the 1990s. Police on Wednesday arrested the father of the suspected shooter of the town for illegal possession of weapons.

The two shootings have sparked calls for change and more tolerance in Serbian society. Thousands of people marched in opposition-led protests in Belgrade and other cities, demanding the resignation of populist government ministers, as well as a ban on TV stations broadcasting violent content and featuring war criminals. More protests are planned for Friday.

Serbia’s populist President Aleksandar Vucic accused opposition parties of using the tragedy for political gain. He has announced plans for a rally of his own at the end of May.

Vucic, a former ultra-nationalist who now says he wants to bring Serbia into the European Union, has faced accusations of promoting hate speech against opponents, stifling free expression with a tight grip on mainstream media and seizing control of all state institutions. He has denied this.