Kennedy, the brother of assassinated US President John F Kennedy and Senator Bobby Kennedy, has been suffering from a brain tumour for some months.
Witnesses report that the ailing 76-year-old suffered a series of violent convulsions. He was treated by paramedics at the scene, and taken away in a wheelchair.
After a battery of tests, doctors treating Kennedy said it appeared the incident was caused by fatigue.
“Senator Edward Kennedy experienced a seizure today while attending a luncheon for President Barack Obama in the US Capitol,” Dr Edward Aulisi of Washington Hospital Centre said.
“After testing, we believe the incident was brought on by simple fatigue. Senator Kennedy is awake, talking with family and friends, and feeling well.”
President rushed to senator\’s side
Kennedy\’s close friends and senators John Kerry and Chris Dodd accompanied him to the ambulance and said Kennedy appeared to be conscious.
“He\’ll be OK, the good news is, he\’s going to be fine – that\’s the best news,” Dodd told reporters, adding Kennedy had said: “\’I\’ll be OK, I\’ll see you later.\'”
Former Vice President Walter Mondale said Kennedy was swapping stories with others at their table when “something happened. I don\’t know what it was, he just stopped.”
“It was really kind of a shock to us all,” he added.
Senator Jay Rockefeller told reporters the new president rushed over to Kennedy\’s side in the Statuary Hall at the US Congress.
“There was a call for silence throughout the room,” he said. “The president went over immediately. The lights went down, just to reduce the heat, I think.”
Obama\’s tribute to Kennedy
Republican Senator Orrin Hatch said that after the scare, Kennedy\’s condition had gradually improved as he was taken to the ambulance.
“I\’m not a doctor, but it looked like a seizure,” he said. “It was painful to him but as he gradually was able to calm down, as he got into the ambulance, he kind of looked over at me and smiled, that old wry smile that I know things are going to be all right.”
He was taken to the Washington Hospital Centre, where staff say he is awake, and being assessed by doctors. He is likely to stay in hospital overnight.
CNN reports the senator is now well enough to talk to members of his family.
Obama acknowledged there were concerns about the health of Ted Kennedy, paying tribute to the senator in his speech to the dignitaries gathered on Capitol Hill.
Malignant brain tumour
“He was there when the Voting Rights Act passed, along with John Lewis who was a warrior for justice.
“And so I would be lying to you if I did not say that right now a part of me is with him. And I think that\’s true for all of us,” Obama said.
There were also early reports that Senator Robert Byrd, at 91 one of the US\’s oldest politicians, had been taken ill during the lunch.
The pair were seated at the same table, and had earlier been shown in television pictures; Kennedy, who has been ill since last May, looked remarkably healthy.
Kennedy underwent surgery for a malignant brain tumour last year after suffering a seizure.
The Massachusetts senator, first elected in 1962, made a high profile endorsement of Obama during the primary campaign and gave an emotional address at the Democratic National Convention in August.