JERUSALEM -- Prime Minister Ariel Sharon of Israel suffered a massive stroke last night and underwent emergency surgery to stop bleeding in his brain, a hospital official said.
Deputy Prime Minister Ehud Olmert assumed Sharon's duties after the prime minister was stricken for the second time in less than three weeks, throwing Israeli politics into confusion at a critical crossroads for the country.
Sharon's spokesman, Ra'anan Gissin, said the prime minister was rushed to Hadassah Ein Kerem Hospital in Jerusalem after feeling ''weakness and numbness" while meeting with party allies at his ranch in the Negev Desert, and underwent an MRI test which detected bleeding in his brain.
Dr. Shlomo Mor-Yosef, director of the hospital, said last night that Sharon had suffered ''a significant stroke" and had been anesthetized and put on a ventilator as doctors prepared him for surgery.
Early today, after more than six hours of surgery, Mor-Yosef characterized Sharon's condition as ''grave," and said doctors expected the operation to take several hours more.
Sharon had been scheduled to check into the hospital today for an operation to repair a tiny hole in his heart, caused by a birth defect, that doctors said allowed a blood clot to escape that caused his first stroke on Dec. 18.
Sharon, a ubiquitous and controversial figure through more than a half-century of Israeli military and political history, in November abandoned the right-wing Likud party he helped found and launched a centrist party that he said would pursue a settlement of the Israeli-Palestinian conflict and try to capitalize on momentum from Israel's withdrawal last summer from the Gaza Strip.
Under Sharon's leadership, the new party, Kadima, was expected to win the largest share of votes in parliamentary elections scheduled for March 28, winning as many as 40 seats in the 120-member Knesset. But if Sharon dies or cannot campaign, the party's future is in question, since it was built around his strong personality and has little established platform or party structure that could anchor the party without him.
Sharon's supporters see him as uniquely able to make bold moves toward an agreement with Palestinians because of his tough and sometimes controversial image as a military commander and longtime proponent of building Israeli settlements in the West Bank and Gaza Strip.
That background has allowed him to build consensus with parts of Israel's right wing and to reassure voters that he will guarantee Israeli security even while ceding territory.
Sharon changed course in recent years and concluded that Israel's occupation of at least some Palestinian territories was untenable, leading to the Gaza pullout in August.
It is unclear if other party leaders, such as the left-wing political veteran Shimon Peres, could pull together an Israeli consensus behind further withdrawals from parts of the West Bank, which Sharon was expected to pursue.
The mood was glum at the Jerusalem hospital, where Sharon's advisers could be seen huddled in a hallway as the operation began and relatives of Sharon began to gather.
The Associated Press cited medical specialists in the United States as saying the type of cerebral hemorrhage Sharon suffered yesterday is extremely serious.
''It's among the most dangerous of all types of strokes," with half of victims dying within a month, said Dr. Robert A. Felberg, a neurologist who directs the stroke program at Ochsner Clinic in New Orleans.
Dr. Philip Steig, chair of neurosurgery at Weill-Cornell Medical Center in New York, was quoted as saying blood thinners administered to Sharon after his first stroke may not have caused the latest stroke but could have made the bleeding worse, and may account for its severity.
Gissin declined to say whether Sharon would be able to campaign or how his party would adjust if he were incapacitated.
''I don't want to speculate about what will be the fate of the Israeli political system after the operation is ended. What's important right now is that you all rest assured that the government of Israel continues to function, to discharge all its responsibilities in security, in foreign policy."
Close to 3 a.m. Israeli time, Gissin emerged from the wing where the operation was going on and told an Arab television station that doctors had found the source of the bleeding and stopped it. He then reiterated that security issues had been transferred smoothly and said, ''Just in case anyone is thinking this is a good time to mess with us, it's not."
The AP, meanwhile, quoted a radical Palestinian leader in Damascus, the Syrian capital, as saying the stroke was a gift from God.
''We say it frankly that God is great and is able to exact revenge on this butcher. . . . We thank God for this gift he presented to us on this new year," said Ahmed Jibril, leader of the Popular Front for the Liberation of Palestine-General Command, a small radical Syrian-backed faction.
But a Palestinian commentator on the Saudi-owned Al-Arabiya network offered Sharon praise as ''the first Israeli leader who stopped claiming Israel had a right to all of the Palestinians' land," a reference to Israel's recent withdrawal from Gaza.
''A live Sharon is better for the Palestinians now, despite all the crimes he has committed against us," said Ghazi al-Saadi.
Gissin said the Cabinet would meet at 9 a.m. today to discuss how the government would proceed. If Sharon is incapacitated, Olmert could continue as acting prime minister until the elections.
President Bush praised Sharon as ''a man of courage and peace" and said he was praying for the Israeli prime minister.
Bush said in a written statement that he and Laura Bush ''share the concerns of the Israeli people . . . and we are praying for his recovery."
The Bush administration helped design what is known as the road map, a plan for restarting negotiations toward creating a Palestinian state alongside Israel. The road map calls on Israel to freeze settlement activity and Palestinians to crack down on militant groups that attack Israel, as preconditions to negotiations. But each side accuses the other of failing to fulfill its obligations.
Sharon was born north of Tel Aviv in 1928 to Russian immigrants, part of a wave of European Jews who established farming communities in what was then British-controlled Palestine.
He has been a figure in most major Israeli events during the course of his career, which began when he was a military officer during the country's war of independence in 1948, in which he battled Jordanian troops in the fight that left West Jerusalem in Israeli hands.
He was a divison commander in the Sinai desert during the 1967 war, in which Israel captured the Sinai peninsula from Egypt, in addition to the West Bank from Jordan and the Golan Heights from Syria.
In 1982 Sharon was defense minister in the government of Menachem Begin, which removed Israeli settlers from the Sinai peninsula and returned it to Egypt as part of a peace deal. But he redoubled his support at that time for Jewish settlement in the West Bank and Gaza Strip -- building a strong following among right-wing proponents of holding onto those territories, a constituency he would later alienate when he spearheaded the pullout from Gaza last summer.
As defense minsiter, Sharon also was a major force in launching the Lebanon War in 1982, an invasion meant to drive out the Palestinian Liberation Organization that left Israel occupying the south of that country until 2000. An Israeli investigation found him ''indirectly responsible" for the massacre of hundreds of Palestinian civilians in the Sabra and Shatila refugee camps by a Christian Lebanese militia in 1982.
But by 2000, he had reemerged as leader of the opposition Likud party, and made a controversial visit to the Temple Mount/Haram Sharif, the site in Jerusalem holiest to both Jews and Muslims. Clashes broke out between Palestinians and Israeli security forces, launching the second Palestinian uprising, which claimed more than 4,000 lives between 2000 and 2004. In 2001 Sharon was elected prime minister.
In recent years he concluded that Israel could not hold onto all of the West Bank and Gaza while retaining its Jewish majority and pushed through the withdrawal from Gaza.
Many Israeli analysts believe he planned to use Kadima as a vehicle for more unilateral withdrawals from parts of the West Bank while retaining large settlement blocs around Jerusalem.
Palestinians were ambivalent toward Sharon's pullout from Gaza, welcoming the first substantive handover of territory to their control but pointing out that Israel still controls land, sea, and air access to Gaza and worrying that by reaching a unilateral settlement Israel will be able to impose its own terms on a final settlement, including the borders of a future Palestinian state.
Globe correspondent Alon Tuval contributed to this story.