At least 413 Palestinians have been killed, including 78 children and 41 women, and nearly 2,300 wounded by Israeli air strikes in Gaza since Saturday, the health ministry of the Palestinian authority reported on Sunday.
The Hamas attack launched from Gaza follows months of rising violence in the Israeli-occupied West Bank, with stepped-up Israeli raids, Palestinian street attacks and assaults by Jewish settlers on Palestinian villages.
Conditions for Palestinians have worsened under the hard-right government of Prime Minister Benjamin Netanyahu. Peacemaking has been stalled for years.
The Gaza Strip has suffered its deadliest day in 15 years in the wake of the unprecedented attack by Hamas into Israel, with Israeli air strikes killing nearly over 400 Palestinians in 24 hours, according to Palestinian officials.
The dead include three-month-old twins killed with their mother and three sisters in an air strike on Saturday in Khan Younis in the southern Gaza Strip, family members said.
At least 10 people were killed in that strike, which destroyed four houses. Rescuers were still trying to find survivors in the rubble on Sunday.
Palestinian men carry a wounded youth following an Israeli air strike at the Erez crossing. AFP
In Gaza, Sabreen Abu Daqqa, who was dug from the rubble of the houses struck in Khan Younis, woke up in hospital to learn three of her children had been killed, two of them had been wounded, and the fate of a sixth was unclear.
"Everything collapsed on top of us. My children were around me," she said, her voice weak as she spoke from hospital. She called out for her children from under the rubble, but heard no reply.
"They began to remove the rubble off me little by little. It took them three hours," she said.
"We are at war,” Israeli Prime Minister Benjamin Netanyahu said in a televised address, declaring a mass army mobilisation. "Not an ‘operation,’ not a ‘round,’ but at war.”
Hamas said the attack was driven by what it said were Israel's escalated attacks on Palestinians in the West Bank, Jerusalem and against Palestinians in Israeli prisons.
Israeli media reported gunbattles between bands of Palestinian fighters and security forces in towns in southern Israel. Israel's police chief said there were "21 active scenes" in southern Israel.
Agencies