"Our sit-in is peaceful and we do not want to destroy public property, but we will stay here until our members are recognised and their salaries paid," union spokesman Khalil al-Zayan said.
Zayan noted that staff recruited by the former Hamas administration had not received any wages for seven months.
"They can no longer support their families, this is unacceptable," he said of the crisis looming over a spring reconciliation agreement between the Islamist group and Palestinian president Mahmud Abbas's Fatah movement.
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Since the striking the deal, Hamas been demanding the new government pay the salaries of the 50,000 civil servants it recruited after its takeover of Gaza in 2007, who took the jobs of 70,000 Fatah employees.
Partial payments of $1,200 each were made to 24,000 Hamas civil servants in late October. But the other 26,000, who work in security functions, have received nothing.
"We've had enough of the false promises. Either the government resigns or it takes all of Gaza's responsibilities, like in the West Bank," Zayan said.
At the end of December, hundreds of Hamas civil servants blocked the entrance to the unity government's headquarters during a visit to Gaza by West Bank-based ministers.