JERUSALEM — Israeli Prime Minister Benjamin Netanyahu vowed revenge Friday for what he described as a “cruel and malicious violation” of the ceasefire agreement after authorities determined that a body released by Hamas as part of the deal was not an Israeli mother of two small boys, as the militant group had promised.

Tens of thousands of students descended upon the central Serbian city of Kragujevac on Saturday in the biggest show of anger yet against the government over a deadly railway disaster in November.