BOSTON (Reuters) - U.S. Sen. Edward Kennedy remained in hospital in Boston on Sunday as doctors tried to determine what caused the patriarch of America's most prominent political dynasty to suffer a seizure on Saturday.

After tests on Saturday, Kennedy's doctor said he was not in any immediate danger and had not suffered a stroke but that more tests were needed. There was no word on Kennedy's condition on Sunday morning.

Kennedy, 76, was airlifted to Massachusetts General Hospital on Saturday morning after being rushed by ambulance to a local hospital near his family's Cape Cod vacation compound.

Kennedy, a leading liberal voice in U.S. politics, is the youngest brother of late President John F. Kennedy and late Sen. Robert Kennedy -- both assassinated in the 1960s. He has served since 1962 when his brother took office as president and is now the second-most senior member of the Senate.

Chris Dodd, a Democratic senator for Connecticut and a long-time friend, said he got encouraging news when he spoke with members of Kennedy's family on Saturday.

"I think the reports sound good," Dodd said on the "Fox News Sunday" television program. "He seems to be doing pretty well. And we just hope for the very best for him and are confident he'll be fine."

It was the second time in seven months that Kennedy has been hospitalized at Massachusetts General. In October, doctors operated on his neck to unclog a partially blocked carotid artery, which supplies blood to the brain.

The blockage was discovered during a routine back and spine examination and, at the time, doctors said a blocked carotid artery can lead to stroke and death.

MANY WELL-WISHERS

All three candidates for November's presidential election -- Democrats Barack Obama and Hillary Clinton and Republican John McCain -- wished Kennedy well and people in Massachusetts expressed shock and concern for their state's senior senator.

Kennedy, who lost the Democratic nomination to run for president to Jimmy Carter in 1980, has endorsed Obama to become the party's nominee to run for president and has actively campaigned for the Illinois senator.

President George W. Bush, a Republican at the other end of the political spectrum from Kennedy, "wishes him a return to good health," said White House spokesman Tony Fratto.

"They've had a good relationship, even with conflicts over policy," Fratto said in the Egyptian resort of Sharm el-Sheikh, where Bush was wrapping up a five-day Middle East tour.

All three of Kennedy's adult children, plus niece Caroline Kennedy and nephew Joseph Kennedy, slipped through a side door at the hospital to avoid reporters. His wife Victoria was there and his colleague, Sen. John Kerry, came to visit.

The first official report on Kennedy's condition came in a statement from his office that he was resting comfortably.

Dr. Larry Ronan, Kennedy's physician who is on staff at Massachusetts General, later said initial tests showed the senator had not suffered a stroke. By evening, Kennedy was watching the Boston Red Sox baseball game on television with his family, Ronan said in a statement.

"Over the next couple of days, Senator Kennedy will undergo further evaluation to determine the cause of the seizure, and a course of treatment will be determined at that time," Ronan said in the statement