Overnight, he was undergoing surgery after being rushed to a hospital in Jerusalem.
Doctors said the 77-year-old Israeli premier has suffered a cerebral hemorrhage. It is Sharon's second stroke in just over two weeks.
Political powers have been transferred to Deputy Prime Minister Ehud Olmert.
Sharon 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.