According to the union, 36 of its members occupied a part of the administration building insisting on the union's recognition, given that it includes more than 15 percent of the factory's workers, which is the minimum stipulated by law.
The protesters said they did not want to obstruction production, so they organised a sit-in in the administration compound.
"We will stay here overnight if nothing changes," a trade union representative said.
The Zastava Oruzje company employs 2,151 people. It produces arms for civilian purposes such as hunting, while its military programme includes automatic weapons, sniper guns, rocket launchers, cannons etc.