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.