At least 15 people were killed and 150 people wounded in an Israeli air strike on a market near Gaza City, medics said, RTÉ reported.
Emergency services spokesman Ashraf al-Qudra said the strike hit a busy market in the battered Shejaiya neighbourhood, which lies between Gaza City and the border with Israel.
The strike came shortly after the Israeli army said it was observing a humanitarian lull that would be in force for four hours from 12pm GMT.
But it said the lull would not apply in areas where troops were "currently operating", in a move denounced as a publicity stunt by Hamas.
Thick black smoke billowed over the site as at least five ambulances raced to the scene where bodies lay strewn on the ground, an AFP correspondent said.
Medics worked frantically to recover the wounded and dead, loading at least five bodies onto stretchers then driving them off in ambulances.
Among those lying on the ground was a man wearing a flak jacket marked "Press", although it was not immediately clear whether he was dead or wounded.
Earlier, the head of the main UN relief agency in Gaza denounced an Israeli strike on one of its schools, which killed 16 people.
"I condemn in the strongest possible terms this serious violation of international law by Israeli forces," said UNRWA commissioner General Pierre Krahenbuhl.
"This is the sixth time that one of our schools has been struck," he said.
Dozens of Palestinians were killed across Gaza today as Israel said it targeted Islamist militants at dozens of sites.
Israeli tank shells and air strikes hit houses and the UN-run school in Jebalya in northern Gaza.
The Israeli military, in an initial response, said militants near the UN facility had fired mortar bombs and Israeli forces had shot back.
Mr Krahenbuhl said about 3,300 people had been sheltering in the school at the moment it was struck.
After 23 days of violence, the Palestinian death toll has risen above 1,300, most of them were civilians.
On the Israeli side, 56 soldiers and three civilians have been killed since the start of the offensive on 8 July.