Israeli forces have conducted an air and ground offensive in Gaza to try to free a soldier captured by Palestinian militants.
Much of the strip was plunged into darkness after Israeli war planes hit a power station.
They also hit three bridges.
Palestinian militants have been erecting barricades and preparing ambush positions in case of ground clashes.
Some Palestinians have been evacuating the area around Gaza airport where the Israeli troops have taken position.
Backed by tanks and helicopter gunships, Israeli troops began moving into Gaza overnight from the Kerem Shalom crossing.
Israel has been preparing to act since the 19-year-old soldier was seized in a cross-border raid on Sunday.
Palestinian militants have demanded that women and children held in Israeli jails be freed in exchange for information on the captured soldier.
Israel says the incursion - the first big one since Israeli withdrawal last year - could last several days.