Updated

Prime Minister Ariel Sharon suffered a massive, life-threatening stroke Wednesday and underwent lengthy surgery to drain blood from his brain after falling ill at his ranch. Powers were transferred to his deputy, Ehud Olmert.

Israeli TV stations reported that Sharon was alive after the first stage of the operation that lasted more than six hours. At daybreak, Army Radio reported that Sharon was undergoing a CT scan to determine if the bleeding in his brain had been stopped.

He was transferred back to the operating room to complete the operation, Shlomo Mor-Yosef, director of Jerusalem's Hadassah hospital, said in an early-morning press conference. The second stage was expected to last several more hours, an estimate that includes time to allow Sharon to respond to medications to stop the bleeding on the brain.

An ambulance brought Sharon to the Jerusalem hospital only hours before the hard-charging, overweight, 77-year-old Israeli leader had been scheduled to undergo a procedure to seal a hole in his heart that contributed to a mild stroke on Dec. 18.

Israel Radio quoted an unidentified Israeli health official as saying that Sharon's prospects of a full recovery were slim.

Sharon's cerebral hemorrhage, or bleeding stroke, came at a time of upheaval among Palestinian factions in Gaza and in the midst of both Israeli and Palestinian election campaigns. Sharon's absence would slow momentum toward peacemaking with the Palestinians and leave a major vacuum at the head of his new Kadima party, which was expected to head a government after the March 28 vote.

In a written statement, President Bush praised Sharon as "a man of courage and peace," saying he and first lady Laura Bush "share the concerns of the Israeli people ... and we are praying for his recovery."

Israeli Chief Rabbi Yona Metzger called on Israelis to read Psalms and pray for Sharon. "We are very, very worried," he said, and prayed for "mercy from Heaven."

Pan-Arab satellite television broadcasters beamed out largely straightforward, nonstop live coverage from outside the hospital where Sharon — a particularly despised figure among many Arabs — struggled for his life.

A radical Palestinian leader in Damascus, the Syrian capital, called the stroke 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," Ahmed Jibril, leader of the Syrian-backed faction Popular Front for the Liberation of Palestine-General Command, a small radical group, told The Associated Press.

Speaking to reporters outside the hospital, Sharon aide Raanan Gissin warned Israel's enemies: "To anyone who entertains any notion to try and exploit this situation ... the security forces and IDF (Israeli military) are ready for any kind of challenge," he said.

But a Palestinian commentator on the Saudi-owned Al-Arabiya network offered Sharon unexpected praise as "the first Israeli leader who stopped claiming Israel had a right to all of the Palestinians' land," a reference to Israeli's recent withdrawal from the Gaza Strip.

"A live Sharon is better for the Palestinians now, despite all the crimes he has committed against us," said Ghazi al-Saadi.

Sharon's personal physician said early Thursday that he expects Sharon to emerge from surgery "safely."

"The prime minister is currently in surgery, it is proceeding properly," said Dr. Shlomo Segev. "We need to wait patiently. I expect him to emerge from it safely."

But Channel 2 TV said the prime minister was suffering from paralysis in his lower body, and medical experts who have not examined Sharon said chances are slim for a complete recovery.

"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 at Ochsner Clinic in New Orleans.

"The fact that he's on a respirator means it's extremely serious," said Dr. Philip Steig, chair of neurosurgery at Weill-Cornell Medical Center in New York.

Dr. Larry Goldstein, director of Duke University's stroke program, said much depends on the extent, location and duration of the bleeding.

"Bleeding in some areas of the brain, if it's caught early enough, you can actually have not a bad outcome," he said.

Sharon was put in an ambulance at his ranch in the Negev Desert after complaining about feeling unwell. The stroke happened during the hourlong drive to Hadassah Hospital in Jerusalem, Dr. Shmuel Shapira of the hospital told Channel 10 TV.

Mor-Yosef said Sharon suffered "a significant stroke," adding that he was "under anesthetic and receiving breathing assistance." A few minutes later, Mor-Yosef emerged to say that initial tests showed Sharon had suffered a cerebral hemorrhage, or bleeding in his brain.

Mor-Yosef said Sharon had "massive bleeding and was being transferred to an operating theater."

Shapira that Sharon was taken to an operating room to drain the blood after suffering what he termed a "massive stroke." Israeli TV said the operation would likely take several hours.

Doctors checking Sharon late last month said he weighed 260 pounds at the time of the first stroke but had since lost more than six pounds and was otherwise in good health. Sharon is about 5-foot-7.

The prime minister had been taking blood thinners since the first stroke to prevent another clot, but such drugs also raise the risk of cerebral hemorrhages, which account for only about 10 percent of strokes. Other possible causes are ruptured blood vessels, an aneurysm, or bulge in a vessel wall that bursts, or even chronic high blood pressure.

Blood thinners may not have caused the latest stroke but could have made the bleeding worse and may account for its severity, Steig said.

Security agents and police spread out around the Jerusalem hospital before Sharon arrived, setting up a security perimeter. Later, they surrounded Olmert's residence in Jerusalem.

Cabinet Secretary Yisrael Maimon said Sharon's authority was transferred to Olmert because the prime minister was under general anesthesia. Olmert was to lead a special Cabinet session on Thursday. Under Israeli law, he will serve as acting prime minister until Sharon can resume his powers.

On Dec. 18, Sharon was taken to Hadassah Hospital from his office after suffering the mild stroke. Doctors said he would not suffer long-term effects, but they discovered a birth defect in his heart that apparently contributed to the stroke.

Sharon had been scheduled to check into the same hospital Thursday for a procedure to repair a tiny hole between the upper chambers of his heart. Doctors said the blood clot that briefly lodged in Sharon's brain last month, causing the mild stroke, made its way through the hole and from there to a cranial artery.

Sharon first came to prominence as an army officer, setting up a unit that fought Palestinian infiltrators in the 1950s. Advancing through the ranks of the army, he served as commander of the Gaza region after Israel captured the territory in the 1967 war, launching punishing raids.

After serving in the 1973 Mideast war, Sharon left the military and entered politics, forging the hardline Likud Party, which came to power in 1977.

As defense minister, he directed Israel's ill-fated invasion of Lebanon in 1982 and was forced to step down by an Israeli commission of inquiry, which found him indirectly responsible for a massacre of Palestinians in two refugee camps by Christian Phalangist soldiers.

Sharon re-emerged as prime minister in 2001, and two years later he reversed his course of decades of support for Jewish settlement construction and expansion in the West Bank and Gaza, promoting a plan for unilateral withdrawal from Gaza and part of the West Bank. The pullout was completed in September.

The withdrawal fractured his Likud party, and he left it to form Kadima with a platform of seeking a compromise for peace with the Palestinians. He was putting together a list of candidates for the parliamentary election when he fell ill Wednesday.

In the March 28 election, Sharon had been expected to face off against Benjamin Netanyahu, the tough-talking former prime minister who recently won the Likud primaries, and Amir Peretz, the union leader who recently unseated veteran Israeli politician Shimon Peres as head of the liberal Labor Party.

Olmert, who could emerge as Sharon's successor in Kadima, would likely have a far tougher time beating either Netanyahu or Peretz than Sharon would have.

According to Israeli law, Olmert as deputy premier assumes the post of prime minister for 100 days if Sharon becomes incapacitated. Then, Israel's ceremonial president would meet with political leaders and choose someone to form a coalition government.