Sen. Edward M. Kennedy (D-Mass.), who was hospitalized in Boston Saturday after suffering a seizure, has a malignant brain tumor, according to a preliminary diagnosis released today by physicians at Massachusetts General Hospital.

A biopsy of a portion of Kennedy's brain identified a malignant glioma as the cause of the seizure, according to a statement by Lee H. Schwamm, the hospital's vice chairman of neurology, and Larry Ronan, Kennedy's primary care physician.

A glioma is the most common type of brain tumor, accounting for more than half of the 18,000 or so diagnosed each year. The prognosis for patients is poor, according to the National Institutes of Health.

The doctors said the usual course of treatment is radiation and chemotherapy.

"Senator Kennedy will remain at Massachusetts General Hospital for the next couple of days according to routine protocol. He remains in good spirits and full of energy," they said.

Kennedy's Senate colleagues learned of the diagnosis after Republicans and Democrats had joined their respective weekly policy lunches. "We stopped what we were doing and we said a prayer," said Sen. John W. Warner (R-Va.).

Sen Patrick J. Leahy (D-Vt.) said Majority Leader Harry M. Reid (D-Nev.) had briefed the Democratic caucus about Kennedy's condition, prompting grave concern among Kennedy's colleagues. "I'm having a hard time remembering a day in my 34 years here I've felt this badly," Leahy said.

Sen. Christopher J. Dodd (D-Conn.), Kennedy's closest friend in the Senate, joined Reid and Sen. John F. Kerry (D-Mass.) at a news briefing after the lunch. He began to speak about Kennedy, predicting that Kennedy would return to the Senate in good health, but stopped and turned away from the microphones. He stood to one side crying while Reid and Kerry spoke to reporters.

White House spokesman Scott Stanzel said President Bush was informed of Kennedy's condition at 1:20 p.m. "Laura and I are concerned to learn of our friend Senator Kennedy's diagnosis," Bush said later in a statement. "Ted Kennedy is a man of tremendous courage, remarkable strength, and powerful spirit. Our thoughts are with Senator Kennedy and his family during this difficult period. We join our fellow Americans in praying for his full recovery."

Kennedy, 76, teamed with Bush during the president's first term to enact Bush's signature No Child Left Behind education legislation, but later became one of his fiercest critics on the invasion of Iraq.

Sen. John McCain (Ariz.), the presumptive Republican presidential nominee, said that "our thoughts and prayers go out to Senator Kennedy and his family. We hope and pray his doctors will be able to effectively treat his condition and that he will experience a full recovery.

"I have described Ted Kennedy as the last lion in the Senate," McCain added, "and I have held that view because he remains the single most effective member of the Senate."

Sen. Barack Obama (D-Ill.), whom Kennedy endorsed for the Democratic presidential nomination, said he and his wife, Michelle, "were saddened to hear the news about Senator Kennedy's condition today, and we plan on doing whatever we can to support him, Vicki, and the entire Kennedy family during this time. Senator Kennedy has been a fighter for his entire life, and I have no doubt that he will fight as hard as he can to get through this.

"He has been there for the American people during some of our country's most trying moments, and now that he's facing his own, I ask all Americans to keep him in our thoughts and prayers," Obama said.

The diagnosis was a sharp turn of events after the weekend's developments. Initial reports Saturday indicated that Kennedy may have suffered a stroke, but that gloomy news soon gave way to more optimistic accounts of Kennedy joking with family, eating a seafood dinner and watching Boston Red Sox games.

Stephanie Cutter, a family spokeswoman, said then that further information on the senator's condition would not be known until Monday. The diagnosis was released today.

Kennedy's doctors said that "he has had no further seizures, remains in good overall condition, and is up and walking around the hospital." They said they will determine Kennedy's course of treatment after further testing and analysis.