Israel launched missile strikes that knocked out power to thousands of Palestinians in Gaza as it pressed an offensive to halt cross-border rocket fire two weeks after its withdrawal from the strip.
The air raids took place just hours after Israel's army fired artillery shells into the Gaza Strip for the first time since the 1967 Middle East war, further aggravating tension on the fifth anniversary of a Palestinian uprising.
Renewed violence has battered hopes that Israel's troop pullout from Gaza, completed earlier this month under Prime Minister Ariel Sharon's plan for "disengaging" from conflict with the Palestinians, would improve chances for peace.
Mr Sharon, locked in a race with rival Benjamin Netanyahu for leadership of the right-wing Likud party, has hit back hard against militants firing rockets into Israel. He wants to counter hardliners' accusations that he has hurt Israel's security by pulling out of Gaza after 38 years of occupation.
His tough response may have have given him a boost in polls showing him taking a commanding lead over Mr Netanyahu.
Before dawn today, Israeli aircraft fired missiles at four militant targets in and around Gaza City, destroying the offices of a leading Fatah militant and two other militant groups, Israeli military sources and Palestinian witnesses said.
They said a fifth missile fired later destroyed a bridge in northern Gaza's Beit Hanoun.
No casualties were reported but the air strikes destroyed two major electrical generators, plunging Gaza City and much of the northern strip into darkness for hours.
The army said it had not deliberately targeted electricity infrastructure. After emergency repairs, at least 35,000 people remained without power after daybreak.
Last night, Israeli artillery fired on what the army said were rocket launching sites in northern Gaza after a rocket landed in a street in a town in southern Israel. A previous rocket barrage drew threats from Defence Minister Shaul Mofaz that militants "will be hit again and again until they understand there are new rules to the game".
However, Israeli officials have given no sign of any immediate intention to send ground forces back into the strip.
Israeli troops also mounted fresh raids in the occupied West Bank, where it has arrested dozens of suspected militants in recent days. Soldiers swept into the towns of Tulkarm and Qalqilya where they sealed the offices of at least two Islamic charities accused of links to Hamas militants, witnesses said.