Israeli tanks have stormed Gaza City, according to Palestinian security sources.
There are no reports yet of casualties.
Earlier, Israeli troops shot dead a 13-year-old Palestinian boy on the edge of the West Bank town of Bethlehem as he and other youths pelted soldiers with stones.
Khifah Obeid was hit in the chest and mortally wounded during a demonstration at the northern entrance to the town, in which youths were protesting at the killing of Palestinians by Israeli forces.
An Israeli army spokesman initially reported an exchange of fire between troops and Palestinian gunmen during the protest.
But after Palestinian witnesses consistently denied there had been shooting at the demonstration, the army admitted it had not been fired upon.
The rally was aimed in particular at protesting the killing of five Palestinian schoolboys by an explosive device last week, and the targeted murder of a militant from the hardline Islamic group Hamas, blamed by Israel for organising the worst suicide bombings in 14 months of violence.
Obeid's death brought the number of people killed in the 14-month Palestinian uprising against Israeli occupation and Jewish settlements to 991, including 779 Palestinians and 190 Israelis.
AFP