The health ministry in Hamas-run Gaza said an Israeli strike on Saturday killed 71 people at the Al-Mawasi camp for war displaced in the south of the Palestinian territory.
Al-Mawasi, near the city of Khan Yunis, was designated as one of the safe zones by Israel after it ordered civilians to evacuate other parts of the Gaza Strip.
The ministry condemned what it called a "brutal massacre by the occupation," referring to Israel. It said 71 people were killed and at least 289 wounded, with additional victims yet to be found.
The Israeli military said it was looking into the reported bombing.
Fleets of ambulances rushed casualties to the Nasser hospital in Khan Yunis, the Kuwaiti field hospital in Rafah and other nearby clinics.
Gaza's civil defence agency said ongoing shelling had prevented its teams from reaching victims in the tent city where tens of thousands have sought refuge.
"There are still many bodies of martyrs scattered in the streets, under the rubble and around the tents of the displaced that cannot be reached due to the heavy shelling of the occupation (Israeli army) that targeted places and tents in the in Al-Mawasi," said civil defence spokesman Mahmud Bassal.
Al-Mawasi had been declared a safe zone by Israel as it pursues its military offensive in other parts of the Gaza Strip.
The UN Palestinian relief agency, UNRWA, has estimated that up to 1.5 million people may now be in the whole Al-Mawasi district.
Suhaib Al-Hams, head of the Kuwait field hospital, called the attack "a real massacre."
He said there were many "severe injuries including amputations and lacerations of internal organs."
"A real disaster is happening now amid the collapse of the healthcare system," he added.
Israel's military reprisal has killed at least 38,345 people in Gaza, also mostly civilians, according to data from Gaza's health ministry.
