Israeli Prime Minister Ariel Sharon last night suffered a "significant" stroke with "massive bleeding" in his brain.
Dr Shlomo Mor-Yosef said Sharon suffered a cerebral haemorrhage, was under general anaesthetic and receiving help breathing.
Cabinet Secretary Yisrael Maimon said Sharon's authorities have been transferred to his vice premier, Ehud Olmert.
In a brief statement outside the hospital, Dr Mor-Yosef said Sharon had suffered "a significant stroke." A few minutes later, Dr Mor-Yosef emerged to say that initial tests showed Sharon had suffered a cerebral haemorrhage, or bleeding inside his brain.
Addressing reporters, Dr Mor-Yosef said Sharon had "massive bleeding and was being transferred to an operating theatre".
Channel 2 TV said Sharon was suffering from paralysis in his lower body. Analysts on Israeli TV stations said he life could be in danger. Channel 2 TV said he was taken into the hospital on a stretcher.
Sharon, aged 77, is over-weight, but doctors checking him after the mild stroke on December 18 found him otherwise in good health.
Since then, his doctors said in a briefing a week ago, Sharon has lost several pounds.
The downturn in Sharon's health comes as Sharon runs for re-election on March 28 at the head of a new centrist party, Kadima, and he enjoys a wide lead in the polls.
The party's strength is centred on Sharon himself, and if he were forced to leave the scene then Israel's political scene would be in turmoil.
On December 18, Sharon was taken to Hadassah Hospital from his office after suffering a mild stroke. Doctors said he would not suffer long-term effects from the stroke, but they found a birth defect in his heart that apparently contributed to the stroke.
Security agents and police spread out around the Jerusalem hospital before Sharon arrived today, setting up a security perimeter.