Officials at Hadassah Hospital confirmed that Sharon was in surgery after tests showed a rise in cranial pressure. Sharon's blood pressure had also risen, according to hospital director Shlomo Mor-Yosef.
Former Prime Minister Shimon Peres said he is "very worried" about Sharon's deteriorating condition.
Two Sharon aides, who were in a meeting elsewhere in Jerusalem, rushed out of the building and were on their way to the hospital, Channel 10 reported. Other reports said acting Prime Minister Ehud Olmert and former Prime Minister Shimon Peres were en route to the hospital.
During a Friday morning press briefing, Sharon's health appeared stable.
"The night passed without change," Mor-Yosef said. "All the parameters that we check: blood pressure, pulse, urine output and cranial pressure, the most important parameters, all these parameters are stable."
Doctors said they could keep Sharon sedated and on a respirator for several more days to give him a chance to recover.
"The logical scenario is that we won't even try to wake him up before Sunday," said Dr. Shmuel Shapira, deputy director of Hadassah Hospital.
"This sedation has very important significance. The goal of the sedation is to lower the oxygen needs of the brain and to allow the brain ... to rest. So certainly until Sunday, and it's possible beyond that, he will be sedated."
On Thursday, as Sharon's sons began a bedside vigil and state media broadcast mournful songs, the hospital's switchboard was flooded with get-well messages. The nation's top rabbis called on Israelis to rush to synagogues and pray for the 77-year-old ex-general, whom many saw as the best hope for peace with the Palestinians.
Sharon's severe illness in the middle of political campaigns – both in Israel and the Palestinian territories – will further complicate Middle East peacemaking efforts, reports CBS News correspondent David Hawkins.
Sharon's deputy, while serving as acting prime minister, but Sharon's dramatic downturn left Israelis fearful.
60 Minutes correspondent Bob Simon says the mood in Tel Aviv is dismal.
"It's a nation in mourning. The radio is playing sad songs, television is non-stop coverage. 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," Simon said.
The Web site of the respected Haaretz daily quoted hospital officials as saying Sharon suffered vast brain damage.
Deputy hospital director Shmuel Shapira told Army Radio that reports of permanent, significant damage were "irresponsible."
Mor-Yosef on Thursday sought to quash widespread rumors that the prime minister was brain-dead. Sharon's pupils were responding to light, "which means the brain is functioning," he told reporters.
"We are fighting for the life of the prime minister, with no compromise," he said. "The main treatment that the prime minister is receiving is a medically induced coma and breathing assistance. The goal of this treatment is ... to allow the brain to recover from the great trauma it suffered."
Dr. Zeev Feldman, a neurosurgeon at Israel's Tel Hashomer Hospital who is not involved in Sharon's treatment, said the test results appeared encouraging.
"I think this is good news. This information that the prime minister is reacting and they got reactions from him to stimulation is really a situation that can show that he is waking up after the operation," Feldman told Channel 2. "This is the first time that we have a positive indications regarding his condition."
However, other neurosurgeons not involved in Sharon's treatment said a full recovery was unlikely after such a massive stroke. Sharon aides said they assume he would not return to work."
I'm worried about the future of this country, about everything in this country," said Rafael Levy, a 42-year-old construction engineer from Tel Aviv.
Sharon underwent seven hours of surgery Thursday at Hadassah Hospital after suffering a brain hemorrhage. He will remain sedated and on a respirator for two to three days to give him time to recover, and then he will be gradually awakened, hospital officials said. His sons, Omri and Gilad, were by his side at the neurological intensive care unit.
Sharon's collapse less than three months before March 28 elections left in limbo his moderate Kadima Party, which had appeared headed for an easy victory.