The death toll reached at least 320 including 62 women and children, according to United Nations officials, with at least 1,400 more wounded.
Israel flattened the Palestinian Interior Ministry building and hit centers of Hamas's social and political network, including a refugee camp on the northern outskirts of Gaza City and a five-story science building at the women's campus at Gaza's Islamic University, the Islamic republic news agency reported.
Hostilities between Israel and Hamas rekindled last week, when a six-month cease-fire ended.
Hamas proved its continued ability to take the fight to Israel on Monday, when one medium-range rocket landed in the Israeli city of Ashkelon, killing an Israeli construction worker there and wounding several others.
He was the second Israeli killed since the beginning of the offensive, and the first person ever to be killed by a rocket in Ashkelon, a city of 120,000.
Two more Israelis died from Palestinian missiles Monday night.
On Sunday, Hamas missiles struck for the first time near the city of Ashdod, twice as far from Gaza as Ashkelon and only 25 miles from Tel Aviv.