Israeli Prime Minister Ariel Sharon is in a critical condition after seven hours of surgery following a major stroke, doctors say.
No other Israeli politician has the stature of Mr Sharon
Doctors managed to stop the bleeding in the 77-year-old's brain, the director of Hadassah hospital in Jerusalem said.
He is to be kept "under heavy anaesthesia and ventilation" for the next 24 hours, Shlomo Mor-Yosef said.
Deputy PM Ehud Olmert has become acting prime minister. Elections scheduled for 28 March are to go ahead as planned.
"The dissolution of parliament and the early elections... are not affected by the prime minister's health condition," Israel's Justice Ministry said in a statement.
Israeli media and political figures are speculating that even if Mr Sharon recovers, he will not continue to lead the country.
The BBC's Jeremy Bowen in Jerusalem says his stroke changes every political calculation in Israel.
None of the other candidates for prime minister can command the same level of trust from Israelis and may not have the will, experience or desire to follow pursue Mr Sharon's plans, our correspondent says.
At the hospital, Mr Mor-Yosef said Mr Sharon had been transferred to a neurosurgery intensive care unit for evaluation and treatment.
He was taken to hospital on Wednesday evening, less than three weeks after being treated for a minor stroke in December.
ARIEL SHARON'S HEALTH
Sharon suffers minor stroke on 18 December 2005
Doctors discover small hole in heart, schedule operation for 5 January
Sharon rushed to hospital one day before scheduled surgery with major stroke
Doctors said the prime minister had suffered a "significant" stroke and subsequently required two separate operations to stop bleeding in his brain.
Mr Sharon, who is seriously overweight, was prescribed blood-thinning drugs after December's stroke and told to diet.
He was due to have an operation on Thursday to close up a small hole in the heart which doctors believed may have caused the earlier stroke.
US President George Bush said in a statement he shared the concerns of the Israeli people "and we are praying for his recovery".
Two senior US envoys who were supposed to travel to Israel to discuss arrangements for Palestinians in East Jerusalem to vote in forthcoming elections cancelled their trip in light of Mr Sharon's illness, as did Japan's Prime Minister Junichiro Koizumi.
Chief Palestinian negotiator Saeb Erekat told the BBC's The World at One programme he was concerned about the possible effect Mr Sharon's absence would have on the peace process.
"I'm really worried that the competition to replace Mr Sharon will be marred with more escalation against the Palestinians whether in the form of assassinations, arrests, incursions of settlements and that worries me a lot," he said.
Newspapers and television in the Arab world have been carrying minute-by-minute updates on the condition of the Israeli leader, a deeply unpopular figure among many Arabs.
Reporting has been largely neutral, observers say.
Mr Sharon, who has been prime minister since 2001, had been planning to run for a third term in office under his newly formed centrist party, Kadima, after quitting the ruling Likud party in November.
Polls had suggested his new party was in the lead ahead of the election in March.
BBC diplomatic correspondent Jonathan Marcus says one of the questions to be answered is whether Kadima can rally smoothly around a new leader, presumably Mr Olmert, will suffer a bruising leadership struggle.