Hadassah Ein Kerem Hospital Director-General, Shlomo Mor Yosef, told reporters that Sharon suffered a "significant stroke" Wednesday night and has been taken to the operation theatre.
A television channel reported that the premier had also suffered from paralysis in his lower body while analysts on TV stations said Sharon's life could be in danger.
Meanwhile, the country's Cabinet Secretary Yisrael Maimon told reporters outside the hospital that deputy premier Ehud Olmert has taken over prime ministerial powers as a result of Sharon's current incapacitation.
Sharon was at his Sycamore ranch when he complained about feeling unwell and was rushed to the hospital, where he was taken to the trauma unit on a stretcher.
A statement from the Prime Minister's bureau said the symptoms were not severe and that he was "fully conscious."
A TV channel reported Sharon was conscious and talking on the phone as he was being taken in his armoured car.
Some media reports said the 77-year-old prime minister's symptoms were similar to those he showed on December 18, when he suffered a mild stroke.
Sharon was scheduled to undergo angioplasty Thursday at the same hospital to block a hole in his heart that was discovered during the stroke he suffered in December.
Since the stroke, he has been receiving blood thinners to prevent a recurrence of the clotting that caused it.
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