Israeli Prime Minister Ariel Sharon has suffered a "significant" stroke and is undergoing an operation, doctors at Jerusalem's Hadassah hospital say.
Officials said the 77-year-old leader was unconscious and had experienced "massive" cerebral bleeding.
The Israeli leader's powers have been transferred to his deputy Ehud Olmert.
It is Mr Sharon's second stroke in just over two weeks. He was due to go into hospital on Thursday to undergo a minor heart operation.
He suffered a minor stroke on 18 December which doctors said could have been the result of a blood clot caused by the hole in the heart.
His doctors said he recovered fully then but required minor surgery for the heart problem.
Sharon resumed his full workload after a stroke in December
Dr Shlomo Mor-Yosef, director of Hadassah Hospital, told reporters at the hospital on Wednesday night that Mr Sharon had "massive bleeding and was being transferred to an operating theatre".
Mr Sharon's press spokesman said he was currently undergoing an operation meant to stop the bleeding in his brain and contain the damage it had caused.
He said "this is a much more serious condition than the previous one", referring to Mr Sharon's first stroke.
"It's not a simple operation," he added.
The prime minister was carried to hospital in Jerusalem from his ranch in the Negev Desert in Israel's south by ambulance - a drive that normally takes more than an hour - instead of by helicopter.
Police and security agents set up a security cordon around the hospital, and also stationed themselves around Mr Olmert's residence in Jerusalem.
The BBC's James Reynolds, outside the hospital in Jerusalem, says however the situation develops now, even if Mr Sharon makes a recovery, there will be serious questions over his health and whether he can continue to lead the country.
Under the constitution Ehud Olmert takes office for 100 days
US President George Bush said in a statement he shared the concerns of the Israeli people "and we are praying for his recovery".
Mr Sharon, who has been prime minister since 2001, is severely overweight.
He is 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 have suggested his new party is in the lead ahead of the election in March.
However, David Horowitz, editor of the Jerusalem Post, said Mr Sharon's health problems changed the political landscape.
"I think already, even in the best case scenario, it's remade Israeli politics," he told BBC News.
He said Mr Olmert did not have the charisma of Mr Sharon or the ability to carry his controversial policies through.
Mr Sharon had indicated he would follow up the withdrawal of Israeli soldiers and settlers from Gaza with what he termed further major concessions - thought likely to involve handing parts of the West Bank over to Palestinians.
The Kadima party "is united solely around Sharon... it will be much less electable without Sharon," he said.
Mr Sharon's spokesman said ministers from the Likud party, who had been due to resign from the cabinet, had reversed their decision so they could help the government through this period.
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