Gaza has come under the most intense shelling since the launch of Israel's offensive, with more than 50 people reported killed in one district, according to BBC.
The deaths occurred at Shejaiya, east of Gaza City, Palestinian medics said. Eyewitnesses spoke of bodies lying in the streets.
A humanitarian truce was agreed in the area, but lasted less than an hour.
Israel earlier said it had expanded its operation - launched 13 days ago - against Hamas militants.
The ceasefire was meant to last from 13:30 to 15:30 local time (10:30-12:30 GMT), according to an Israeli military spokesman.
But a BBC team on the ground reported an exchange of fire less than an hour after the truce began.
Both sides blamed the other for violating the truce.
The death toll continued to rise at the weekend, with the number of Palestinians killed now more than 400 since the operation began, according to Palestinian health officials.
The majority of those killed are civilians, the UN says.
