Tens of thousands of migrants in recent days have crossed onto a tiny peninsula on Morocco’s coast governed by Spain, aggravating tensions over a border that has long troubled the relationship between the two governments.

Nearly three dozen people died trying to enter Ceuta, the head of the city’s local government said on Frida, and by Friday afternoon, the vast majority of those who came had returned to Morocco.

The peninsula, Ceuta, is one of only two cities on the African mainland that are officially part of Europe. Thousands of migrants over the years have scaled a 20-foot-high border fence into Ceuta or entered the territory by sea in mass crossing episodes.

But so many migrants made the passage on Thursday that it prompted Spain to mobilize its military and ignited backlash from the country’s right-wing opposition. That faction has accused the government of Prime Minister Pedro Sánchez of being lax on border control.