Sharon's blood pressure also rose and one of his brain lobes expanded slightly, said Dr. Shlomo Mor-Yosef, the director of Hadassah Hospital, where Sharon is being treated. Sharon's aides rushed to the hospital to be with him during the surgery, his second in two days.
"It was decided to bring the prime minister to the operating room in order to deal with these two issues, to drain the bleeding and to decrease the intracranial pressure," he said.
The fact that Sharon went back into surgery is seen as a very bad sign, and is underscored by the news that several top aides left meetings to come to the hospital, reports CBS News correspondent Allen Pizzey.
Shimon Peres, Israel's elder statesman and a Sharon ally, said he was "very worried."
The surgery Friday followed a seven-hour operation Sharon underwent Thursday morning after he suffered a massive brain hemorrhage. Doctors had put him in a medically induced coma to give his body time to heal, but most outside experts said his chances for recovery were slim.
On Friday morning, doctors sent him back for a brain scan to monitor his condition. After the scan, he was rushed back into the emergency room, Mor-Yosef said.
Sharon's sons, Omri and Gilad, were camped out in a room next door to their father's at the neurological intensive care unit.
Dr. Yonathan Halevy, a senior doctor at Jerusalem's Shaarei Zedek Hospital who is not treating Sharon, said he was also worried about the Israeli leader's condition.
"The fact that the bleeding has resumed is a sign of a significant deterioration," he told Israel TV.
Outside medical experts said bleeding from the stroke may have led to interference of the drainage of the cerebral spinal fluid that bathes the brain, or he may have developed inflammation and fluid leakage within the substance of the brain.
Sharon's sudden, grave illness left his ambitious peace agenda in doubt and stunned Israelis, who were grappling with the likelihood that the man who dominated politics in the regions for decades would never return to power. "Between hope and despair," read the banner headline in the Maariv daily.
"It's a nation in mourning. The radio is playing sad songs, television is non-stop coverage," reports 60 Minutes correspondent Bob Simon. "It's mourning and it's shock, and it's very comparable to my mind to the death of [former Israeli Prime Minister Yitzhak] Rabin 10 years ago.
Aides to Sharon said they were working on the assumption he would not return to work.
Sharon's supporters prayed for his recovery. Sephardic Chief Rabbi Shlomo Amar advised Israelis of which psalms to read as part of their prayers for Sharon.
Rabbi Shmuel Rabinovitch of the Western Wall said he received dozens of e-mails praying for Sharon's health that he printed out and stuck in the cracks of the holy site. Callers from as far away as Venezuela and the United States asked for advice in praying for Sharon, he said.
Svetlana Kremitsky, a hospital worker who brings food to the patients in Sharon's ward, said the hospital was filled with worry.
"You can feel it in the air, we're all concerned," she said.
Sharon's deputy, Ehud Olmert, has
Doctors said it would take time to determine how much damage was caused by the widespread stroke Sharon suffered Wednesday night adding that media reports of permanent, significant damage were irresponsible.
Sharon's collapse less than three months before national elections also left his Kadima party, which he formed in November, in limbo.
In the short-term, Israelis appeared to still be supporting Kadima. A poll published in the Yediot Ahronot daily Friday found that an Olmert-headed Kadima would win 39 of 120 parliament seats, the most of any party and slightly less than the party polled under Sharon.
The dovish Labor Party would get 20 seats, and the hard-line Likud, which Sharon left to form Kadima, would capture 16 seats, according to the poll. The poll of 500 people was taken Thursday. It had an error margin of 4.4 percentage points. Some pollsters said the results might be influenced by sympathy for Sharon, and could change during the three-month campaign.
Peres would net 42 seats as head of Kadima, but some analysts said it was unlikely he would be chosen to lead the new party. He met with Olmert on Friday, but did not give details.
"We will know how to continue Israel's policy ... to continue Ariel Sharon's policies," Peres told reporters.
Palestinians reacted to the fall of their longtime enemy with a mix of glee and apprehension. Some Palestinian leaders worried that Sharon's illness could derail their Jan. 25 parliamentary elections. "We are watching with great worry at what might happen if he is harmed," said Palestinian leader Mahmoud Abbas, who called Olmert to express wishes for Sharon's recovery.
Some Palestinian children gave out sweets in the Gaza Strip at news of Sharon's illness.
Foreign leaders, who embraced Sharon following his unilateral pullout from the Gaza Strip last year, also expressed concern.
U.S. Secretary of State Condoleezza Rice praised Sharon as "a man of enormous courage," and British Foreign Secretary Jack Straw said he was praying for a miraculous recovery.
But Christian broadcaster Pat Robertson suggested thatfor "dividing God's land."
Sharon had been expected to win the March 28 election in a landslide as head of Kadima, which he formed after bolting Likud last year. Many hardline Likud lawmakers tried to torpedo the Gaza withdrawal and Sharon formed Kadima to free his hands to make further peace moves with the Palestinians.