Surgeons at Bethesda Naval Hospital yesterday removed a tumor about two inches in diameter from President Reagan's lower intestine, or colon, during a 2-hour, 53-minute operation that turned up no immediate evidence that the tumor had become cancerous.

The surgeons said that the operation, which also removed about one third of the president's colon, went well and that Reagan, 74, should be able to make a full recovery in a few weeks, returning to as vigorous a life as he has traditionally led. However, tests on the removed tumor, which may still reveal that it was malignant, will not be completed until Monday.

"It gives me great pleasure to tell you that the president of the United States is now . . . doing beautifully. Our president is doing very, very well," Dr. Dale Oller said after the operation. Oller, the naval hospital's chief of surgery, led a six-man surgical team that worked on Reagan. "I would anticipate he will start doing paper work first thing in the morning," Oller said.

Just before Reagan underwent surgery, he signed a letter empowering Vice President Bush to act as president. Bush cut short a weekend in Maine to return to Washington.

About 7:45 last night, more than five hours after the operation ended, the White House announced that Reagan was resting comfortably in the recovery room, chatting with nurses and doctors and declaring, "I feel fit as a fiddle." As Oller stood at the foot of his bed, the president praised his doctors, saying "These fellows did a good job."

Reagan's vital signs were good and, according to White House spokesman Larry Speakes, he was "generally without pain." Reagan felt well enough, Speakes said, that at 7:22 p.m. he signed a letter in effect reclaiming his presidential powers from Bush. When it was suggested that Reagan might want to wait a while before rescinding the transfer of power, the president said, "Gimme a pen," according to Speakes.

Reagan was expected to remain in the recovery room overnight and return to his suite at the hospital this morning.

Earlier, Dr. Steven Rosenberg, chief of surgery at the National Cancer Institute and a member of the medical team attending the president, said: "Everything you've heard about the president's vigor on the outside is certainly corroborated by what we saw on the inside."

The doctors' observations during the surgery are a favorable sign for the president, but they cautioned that tumors of this type and size have a greater than 50 percent chance of being malignant.

Slices of the tumor, which was confirmed to be a villous adenoma, the type of intestinal tumor most likely to be cancerous, are being prepared for a detailed microscopic examination that will not be complete until around noon Monday. That analysis should confirm whether the growth had become malignant.

A preliminary test of tissue removed during Friday's diagnostic procedure revealed no malignant cells. But because the sample was taken from the outside of the tumor, the possibility remains that cancerous cells could be deeper inside.

Just 11 days ago the president's brother, J. Neil Reagan, 76, underwent a similar operation for cancer of the colon. "I was home on the fifth day after the operation. He's not going to be any different than me," Neil Reagan said from his home in Rancho Santa Fe, Calif.

Because colon cancer tends to run in families, doctors who knew of Neil Reagan's diagnosis would have had a basis for thinking the president was at a slightly higher than normal risk of having the same disease.

The procedure performed on the president was the same as if the tumor were already known to be malignant. The surgeons removed a two-foot-long segment of Reagan's colon along with associated tissues, which contain lymph nodes that are also being examined for evidence of any spread of malignancy, which may or may not have occurred if the tumor was cancerous.

Even if the tumor proves to have been malignant, the doctors said the surgery that was done yesterday should be all the treatment Reagan needs.

Reagan's doctors said they had rejoined the cut ends of his intestine, reestablishing a normal pathway for bodily wastes. They said that when the president is able to resume a regular diet, his bowel function should be virtually normal. Reagan will be fed intravenously at first, then will be able to resume taking liquids and, soon afterward, begin to take solid foods.

Statistics on a large number of patients with similar tumors indicate that even if the president's growth, officially described as five centimeters across, proves to have contained malignant cells, Reagan would have a better than 90 percent chance of surviving five years, as long as none of the cells has invaded the colon wall. If Monday's report shows that invasion has begun, the five-year survival rate drops to only 40 to 50 percent.

Rosenberg said that, given what was known yesterday, there was a "reasonable likelihood" that the operation will prove "curative." Reagan, he said, "should return to the exact state he was in prior to this operation."

Oller said Reagan would probably be out of the hospital in a week to 10 days and be able to return to vigorous activity in six to eight weeks. He even predicted that the president would be able to stick to a previously planned schedule that calls for him to go to his California ranch Aug. 14 and go horseback riding.

To perform the operation, surgeons cut into Reagan's abdomen, exposing the portion of the lower intestine, or colon, that runs from its juncture with the small intestine at the lower right, near the appendix, upward to a point near the rib cage. From there the colon runs horizontally to the left side and then down to the rectum. The tumor was at the beginning of the colon, in a segment called the cecum.

The surgeons removed a short segment of the small intestine, the adjoining cecum and about two feet of the ascending colon. They then sewed together the remaining cut ends.

Surgeons not connected with the Reagan case said this was a normal procedure. This is because the right-hand half of the colon is supplied by a single set of blood vessels. A different set supplies the left side. Because malignant cancer cells can spread through the blood vessels, removal of all or most of the right colon ensures that, if there had been any spread, the surgeons have removed the tissue to which it would have gone first and established new tumors.

The spread of cancer cells from a malignancy in the colon is also highly likely to go to the liver. If this occurs, the survival chances drop to almost zero. Reagan's doctors, however, said they looked at his liver and saw no sign of abnormality. They said there was no need even to remove a small piece for laboratory analysis.

To reduce normal post-operative pain, a newly developed device that drips tiny doses of morphine directly into the body was inserted into Reagan's back. It delivers repeated doses of morphine to a region near the spinal cord, blocking pain without the need of conventional larger doses that may cloud the mind.

By 4 p.m., about an hour and 20 minutes after the operation ended, Reagan was awake in the recovery room and conversing comfortably. He was also receiving an intravenous drip intended to supply sugar for energy and additional fluids and other substances that maintain normal blood pressure.

Reagan also has a tube through his nose that extends into his stomach to suck out any fluids or gases that may accumulate and make him uncomfortable.

During the recuperative period Reagan's diet will be controlled so that nothing he eats or drinks will prove too great a challenge for his intestinal system. Any abdominal surgery, even as simple a procedure as an appendectomy, disrupts the normal activity of the intestines, which rely on rhythmic waves of muscular contraction to move digested food through some 20 feet of small intestine and the normal six feet of the large intestine.

Post-operative patients often experience abdominal cramps as the muscular contractions resume, often in the wrong direction or even in opposing directions.

In addition, Reagan's doctors do not want to subject the stitches now joining the cut ends of intestine to the mechanical forces of moving wastes before they are healed.

Reagan's physical activity will be similarly controlled. Oller said the president would probably be allowed to get out of bed and sit in a chair today. Over the next few days he would be allowed progressively more strenuous physical activity such as walking around his suite, which includes auxiliary bedrooms, a conference room and a kitchen.

Although the president is expected to be able to return to the White House in a week or 10 days and resume normal office duties, he will be required to have several periods of rest each day for the first few weeks, the doctors said.

Rosenberg said there was a chance that new polyps could develop in Reagan's intestine. "Many people who develop polyps develop recurrent polyps," he said. "And there's no question that the president should undergo from now on regular examination, either by colonoscopy the procedure that was done Friday or barium enema, for recurrent polyps."

Rosenberg, who is not Reagan's primary doctor, said he would expect the president should have a repeat colonoscopy in six months and additional ones annually thereafter.

The first hint that Reagan had any intestinal problems occurred in May 1984 when a routine medical exam, which included an examination of the rectum and nearby portion of the colon, turned up a small polyp. It was found to be benign. A second polyp was found this March.

Some doctors have expressed surprise that doctors did not recommend a full colonoscopy, which examines the entire six-foot length of the colon, when the first polyp was found, but the president's doctors yesterday said they believe that no mistake was made in waiting until Friday to perform the procedure. That is when the third polyp, the one found to be a villous adenoma and removed yesterday, was discovered.

Reagan spent Friday evening conferring with his doctors and his aides and, according to Speakes, he sat reading in his bed until around midnight.

Yesterday morning, as he signed the letter that transferred presidential powers to Bush, the president turned to Nancy Reagan, who had arrived from the White House earlier in the morning, and said, "But you're still my First Lady."

Reagan left his room at 11:15 a.m. and was wheeled to an operating room just down the hall. Mrs. Reagan walked beside the president, holding hands with him.

When they reached the sterile zone, Mrs. Reagan gave her husband a kiss and each said to the other, "I love you." Reagan was taken into the operating room and Mrs. Reagan went to a waiting room. The operation began at 11:48 a.m.