JERUSALEM - Waves of Israeli airstrikes on security compounds in the Hamas-ruled Gaza Strip killed at least 225 people yesterday, sending tremors through the Middle East and inflaming the Israeli-Palestinian conflict at a time of transition in Washington.
The start of what could be an open-ended campaign marked the deadliest day of fighting in Gaza in decades, coming after militants in the coastal territory had stepped up attacks on Israel, firing scores of rockets and mortar rounds after the expiration of a shaky truce.
Israel warned it might go after Hamas' leaders, and militants kept pelting Israel with rockets - killing at least one Israeli and wounding six.
Hundreds of Israeli infantry and armored corps troops headed for the Gaza border in preparation for a possible ground invasion, military officials said, speaking on condition of anonymity under army guidelines.
Prime Minister Ehud Olmert said late yesterday that the goal was "to bring about a fundamental improvement in the security situation." He added, "It could take some time."
Mu'awiya Hassanein, chief of emergency services in the Gaza Strip, said that 225 people had been killed and 750 wounded in the Israeli attacks, which continued into the night. Gaza militants fired fresh salvos or rockets at southern Israel, killing one man in the town of Netivot and wounding several others.
The offensive began eight days after a six-month truce between Israel and the militants expired. The Israeli army says Palestinian militants have fired some 300 rockets and mortars at Israeli targets over the past week, and 10 times that number over the past year.
The surge in violence also came at a time of political flux in the U.S. and Israel, underlining the explosive potential of the Arab-Israeli conflict just as President-elect Barack Obama prepares to take office and weeks before Israelis go to the polls for national elections.
Analysts said that it was too early to tell whether the deadly eruption would complicate peace efforts by a new U.S. administration, and that much depended on how powerfully the bloody scenes from Gaza would reverberate in the region, where angry street protests erupted in several Arab countries.
Yossi Alpher, an Israeli strategic analyst, said he expected the army action "to return us to some sort of status quo ante: If it succeeds we're back to some sort of cease-fire, and if it fails we're back to some sort of tit for tat."
A spokeswoman for Obama, who is vacationing in Hawaii, said he was closely monitoring events in Gaza, and an aide said the president-elect had discussed the Mideast situation in a phone conversation with Secretary of State Condoleezza Rice.
The White House put the blame for the violence on Hamas, and pointedly avoided calling on Israel to halt its military action. "Hamas' continued rocket attacks on Israel must cease if the violence is to stop," said While House spokesman Gordon Johndroe, urging Israel "to avoid civilian casualties as it targets Hamas in Gaza."
The leader of the Hamas government in Gaza, Ismail Haniyeh, said in a statement that "Palestine has never witnessed an uglier massacre." Later, in a televised speech, he vowed to fight Israel. "We say in all confidence that even if we are hung on the gallows or they make our blood flow in the streets or they tear our bodies apart, we will bow only before God, and we will not abandon Palestine," he said.
In Damascus, Syria, Hamas' supreme leader, Khaled Meshal, said in an interview with Al-Jazeera television that he was calling for a new Palestinian intifada against Israel, including the resumption of suicide attacks within Israel for the first time since 2005. Hamas, he said, had accepted "all the peaceful options, but without results."
The Israeli strikes hit more than 100 targets, the army said, including security compounds and police buildings across the Gaza Strip - some in crowded neighborhoods - leaving scores of dead and injured, mostly uniformed officers but also at least 15 civilians. Smoke rose over Gaza as explosions shook the city. "It was like an earthquake," said Ahmed Ghannam, a shopkeeper.
At the main police headquarters in Gaza City, which was hit during a graduation ceremony for new officers, the bodies of more than a dozen men were strewn on the ground next to the wounded. One injured officer raised his index finger, chanting the Muslim affirmation of faith: "There is no God but Allah, and Muhammad is his messenger." The dead included Gaza's police commander, Tawfik Jaber.
There were scenes of pandemonium at Shifa Hospital, which was overwhelmed with casualties, some of which were treated on the floor and in corridors. Bodies wrapped in blankets were piled near the morgue. A man emerged from the building carrying a dead child, and the body of a girl lay on a stretcher.
The Arab League initially called an emergency meeting for today in Cairo with all the foreign ministers from the member states but later postponed it until Wednesday to give ministers time to respond.
Governments that dislike Hamas, like Egypt's, Jordan's and the Palestinian Authority in the West Bank, are in a delicate position. They blame Hamas for having taken over Gaza by force 18 months ago in the aftermath of its election victory in the Palestinian parliament, and they oppose its rocket fire on Israeli towns and communities.
But the sight of scores of Palestinians killed by Israeli warplanes outraged their citizens, and anti-Israel demonstrations broke out across the region.
President Mahmoud Abbas of the Palestinian Authority angrily condemned the Israeli airstrikes. Egypt, worried about possible efforts by Palestinians to enter the country, has set up machine guns along the Gaza border. But yesterday, it temporarily opened the Rafah border crossing in order to allow the wounded to be brought to Egyptian hospitals for treatment.
The Israeli authorities put towns near the Gaza Strip on alert, and sirens warned of incoming rockets as residents took cover in shelters.
Israeli Defense Minister Ehud Barak said the Israeli operation would be "deepened and broadened as much as is necessary."
"It will not be easy, and it will not be short," Barak added, vowing to "fundamentally change the situation" and halt the attacks from Gaza.
The Israeli government had been under mounting pressure to respond to escalating rocket strikes in recent days, which caused no serious casualties but terrorized residents in southern towns and farming communities.
Two key figures in the Israeli Cabinet, Barak and Foreign Minister Tzipi Livni, are candidates for prime minister in the election campaign, facing a challenge from the hawkish leader of the Likud party, Benjamin Netanyahu.
The New York Times and the Associated Press contributed to this article.