Israeli forces on an offensive in Gaza killed 20 Palestinians in air strikes and clashes with militants today in the worst violence since the Jewish state quit the strip a year ago.
Gunmen also killed an Israeli soldier in northern Gaza, the Israeli army said.
Prime Minister Ehud Olmert ordered tanks to push deeper into Gaza overnight after militants from the ruling Hamas movement fired rockets into a major Israeli city for the first time.
The incursion expands an offensive that began last week with the main goal of bringing home captured soldier Gilad Shalit and puts pressure on the Hamas government, already on the brink of collapse because of a Western aid embargo.
Palestinian Interior Minister Saeed Seyam of Hamas called on all the government's security services to "confront this incursion and cowardly Zionist aggression". Most forces are loyal to moderate President Mahmoud Abbas of Fatah.
Among the deadliest attacks was an Israeli air strike near the town of Beit Lahiya in northern Gaza killed six Palestinian civilians, witnesses and medics said.
The army said it had carried out two air strikes against armed militants in the area at the time, killing four gunmen. A spokeswoman said she was unaware of civilian deaths.
A night-time air strike killed at least three Palestinians near Beit Lahiya, a Palestinian security source said. The army said it fired at gunmen.
Beit Lahiya was also the scene of the heaviest ground fighting, where gunmen from various factions fired anti-tank rockets from narrow alleyways in running battles with Israeli troops backed by tanks and helicopter gunships.
An Israeli battalion commander said his forces had killed seven gunmen with sniper and tank fire.
Two soldiers were wounded, the army said.
Before night fell, Beit Lahiya's streets were largely deserted as frightened residents sheltered indoors. "Israeli tanks are outside our house. Children are screaming and the house is shaking. We are caught in the crossfire," one woman told a local radio station.
Aircraft also launched missile strikes near the southern town of Khan Younis.