JERUSALEM — Prime Minister Ariel Sharon underwent emergency brain surgery today after doctors detected bleeding and increasing pressure, and he would undergo a brain scan after the operation, doctors said.

Israeli TV reported that Sharon's surgery ended after three hours, but Hadassah Hospital official Ron Krumer denied that, saying the 77-year-old premier was still in the operating room.

Sharon suffered a massive stroke with widespread bleeding in his brain Wednesday, and today's surgery was the second in two days. He was rushed into the operating room after doctors detected bleeding in the brain, increased cranial pressure and rising blood pressure.

One of Sharon's brain lobes expanded slightly, said hospital director Dr. Shlomo Mor-Yosef.

Wednesday's stroke was Sharon's second since a mild one Dec. 18 and occurred the night before he was scheduled to undergo a procedure to repair a hole in his heart. That tiny hole contributed to the earlier stroke.

Shimon Peres, Israel's elder statesman and a Sharon ally, said he was "very worried."

The surgery today followed a seven-hour operation Thursday morning after Sharon suffered a massive brain hemorrhage. Doctors had put him in a medically induced coma to give his body time to heal, but most outside experts said his chances for recovery were slim.

This morning, doctors sent him back for a brain scan to monitor his condition. After the scan, he was rushed back into the emergency room, Mor-Yosef said.

Sharon's sons, Omri and Gilad, were camped out in a room next door to their father's at the neurological intensive care unit.

Dr. Yonathan Halevy, a senior doctor at Jerusalem's Shaarei Zedek Hospital who is not treating Sharon, said he was also worried about the Israeli leader's condition.

"The fact that the bleeding has resumed is a sign of a significant deterioration," he told Israel TV.

Outside medical experts said bleeding from the stroke may have led to interference of the drainage of the cerebral spinal fluid that bathes the brain, or he may have developed inflammation and fluid leakage within the substance of the brain.