Israel’s military told some 1 million Palestinians on Friday to evacuate northern Gaza and head to the southern part of the besieged territory, an unprecedented order applying to almost half the population ahead of an expected ground invasion against the ruling Hamas militant group.

The U.N. warned that so many people fleeing en masse — with just a 24-hour deadline — would be calamitous. Hamas, which staged a shocking and brutal attack on Israel nearly a week ago and has fired thousands of rockets since, dismissed it as a ploy and called on people to stay in their homes.

The evacuation order, which includes Gaza City, home to hundreds of thousands of Palestinians, sparked widespread panic among civilians and aid workers already running from Israeli airstrikes and contending with a total siege and a territory-wide blackout.

“Forget about food, forget about electricity, forget about fuel. The only concern now is just if you’ll make it, if you’re going to live,” said Nebal Farsakh, a spokesperson for the Palestinian Red Crescent in Gaza City, as she broke into heaving sobs.