Israeli tank shells killed 18 civilians in Gaza on Wednesday including 13 members of one family, Palestinian officials and residents said, in one of Israel's deadliest strikes in the territory in months.
Witnesses said shells struck at least seven houses in the northern town of Beit Hanoun, killing people as they slept.
Seven children and four women were among those killed, said Palestinian Health Ministry spokesman Khaled Rabi. The ministry said 54 people had been wounded.
The Israeli army said it fired artillery shells at northern Gaza in response to militant rocket fire and was checking reports of Palestinian casualties.
Soon afterwards Defence Minister Amir Peretz ordered the army to stop shelling Gaza.
"It is the saddest scene and images I have ever seen. We saw legs, we saw heads, we saw hands scattered in the street," 22-year-old Attaf Hamad, a witness, said.
Residents said 13 members of one family had been killed in Beit Hanoun. They owned four of the houses that were hit.
The Israeli withdrew from Beit Hanoun on Tuesday following its biggest offensive in Gaza in a year.
That week-long assault, designed to stop militants firing rockets at the Jewish state, had killed 52 militants and civilians.
Prime Minister Ismail Haniyeh of the ruling Hamas militant faction called for an immediate session of the United Nations in response to the Gaza attacks.
President Mahmoud Abbas, a moderate, called the shelling a "horrible and ugly massacre".
Haniyeh also called for a suspension of talks with Abbas's Fatah movement on forming a unity government that Palestinians hope will lead to a lifting of sanctions imposed on Hamas over its refusal to recognise Israel.
In the occupied West Bank, Israeli soldiers killed four gunmen and a civilian during an early morning raid near the northern city of Jenin, Palestinian security officials said.
A senior Hamas official urged Palestinians to carry out attacks inside Israel in response to the Gaza shelling. The Islamic Jihad group vowed to carry out suicide bombings.
"The reaction is coming. (Israel) prepare the coffins and black body bags," Nizar Rayan, a hardline Hamas leader shouted to an agitated crowd outside a morgue in Beit Hanoun.
"Revenge for the blood of martyrs is coming ... We urge our mujahideen to resume bombing attacks in Jaffa, in Haifa, in Ashdod -- every place in our homeland."
Israel had pulled its forces out of Gaza last year after a 38-year occupation.
But tensions have flared since Hamas rose to power after winning Palestinian elections in January, and gunmen captured an Israeli soldier in a cross-border raid in June.
Pools of blood lay in the streets of Beit Hanoun. Pieces of flesh and the small sandals of children were strewn about.
Residents, who said they were not aware of any rockets being fired at Israel from the area overnight, rushed to look for relatives. Some fainted when they saw the carnage of dead and wounded.
One man wailed as he tried to find his son.
"Where is my son?" he screamed. Neighbours did not have the heart to tell him his son was dead.
Unidentified gunmen opened fire on the European Union office in Gaza City soon after the shelling, Palestinian security sources said. They said there were no reports of casualties.
In the West Bank, Palestinian security sources said Israeli troops shot dead four gunmen and a civilian in a raid near Jenin.
The gunmen were from the militant Al Aqsa Martyrs Brigades group, part of Abbas's Fatah faction.
The Israeli military said troops had opened fire during an operation to arrest wanted gunmen in the village of Yamoun.