Prime Minister Ariel Sharon suffered a major setback on Saturday when a clot in an artery in his abdomen closed off the blood supply to part of his large intestine and required the surgical removal of a third (50 centimeters) of it. There were no complications during the surgery, according to Hadassah Medical Organization director-general Shlomo Mor-Yosef. After the three-hour operation, performed by senior surgeons at Hadassah-University Hospital in Ein Kerem, Sharon was in "critical but stable" condition, out of immediate danger and again attached to a respirator after he had been breathing on his own for weeks since his severe hemorrhagic stroke on January 4. He is now recovering in the general intensive care unit on the ground floor after having spent five weeks in the seventh-floor neurosurgical intensive care unit. The sudden change in location required Hadassah to implement new security arrangements. Acute occlusion of a branch of the superior mesenteric artery is usually due to a local embolism (clot) that originates in the blood vessel; the branch is about three centimeters in diameter. When blood does not reach the colon, it causes tissue to die in a process called necrosis. This condition is life threatening because dead tissue poisons the body and requires urgent surgical opening of the blood vessel and removal of the necrotic tissue. According to Hadassah, first signs of a problem occurred at 4 a.m. when Sharon's blood pressure dropped suddenly and his abdomen swelled. He was taken from a computerized tomography (CT) scan, which showed necrotic tissue, and then rushed to the operating room, where surgery was performed by Prof. Avi Rivkind, chief of surgery and head of the trauma department, and senior surgeon Prof. Alon Pikarsky. The anesthesiologists were Dr. Yitzhak Gozal and Dr. Yoram Weiss, who were involved in Sharon's brain operations. This was the seventh surgery - the third major one - performed on Sharon since January 4. Sharon's feeding tube, inserted surgically a few weeks ago into his stomach to reduce the risk of infection, was removed during Saturday's operation. He will be fed intravenously in the meantime, but doctors plan eventually to reinsert the tube. Mor-Yosef said the sudden complication was not unusual in comatose patients and that the surgery was a "relatively simple procedure," leaving the prime minister's long coma as his major medical problem. Officials in the Prime Minister's Office said that despite Sharon's condition, the weekly cabinet meeting, as well as a Kadima political event, will take place as scheduled on Sunday. The officials said that Acting Prime Minister Ehud Olmert was updated about Sharon's condition Saturday morning, and was kept constantly abreast of the situation. When the emergency became known, Sharon's sons Omri and Gilad rushed to the hospital. They were later joined by top aides Dov Weisglass, Yisrael Maimon, Shalom Turgeman, Ilan Cohen, Assi Shariv, Ra'anan Gissin, Lior Horev and others. One official said that this group has taken turns sitting vigil at the hospital to comfort Sharon's sons, meet guests who have come to express their good wishes and just out of a personal, emotional need just to be there. Prof. Jonathan Halevy, director-general of Jerusalem's Shaare Zedek Hospital and veteran internal medicine specialist, told The Jerusalem Post Saturday night that such emergency complications are quite common in elderly, comatose patients, and that there is nothing one can do to prevent them. Sharon's obesity made it much more likely to occur. Surgeons look at blue sections of the large intestine, which indicate lack of blood supply, and remove them along with small bits of healthy tissue at the ends. Halevy added that the branch of the artery affected by the clot was the distal ileocolic artery and that it resulted from "low-flow states" that led to necrosis. The part of the colon that was surgically removed was near the appendix in the lower-right quadrant of the abdomen. Herb Keinon contributed to this report.