"For more than 40 years now the Kennedy family has inspired Americans to public service, strengthened our faith in the future and moved our nation forward. Through it all, they have suffered much and given more," the President said.
"We hope the families of these three fine young people will feel the strength of God, the love of their friends and the prayers of their fellow citizens."
The Internet has been inundated with prayers and condolences since the news broke. America Online Inc, the world's biggest online service provider, said tens of thousands of messages had poured in at a rate which had not been seen since the death of Diana, Princess of Wales.
One Internet surfer said: "My heart goes out to ... the Kennedys. ... They have lost the classiest man and the best representative of their clan."
Said another: "I feel the same way I felt when Diana died."
The Pope said he was praying for the whole Kennedy family. "This is only the latest tragedy that family has suffered," said his spokesman.
In Australia the Deputy Prime Minister, Tim Fischer, said in a television interview: "Australian people will be saddened by this news as it unfolds from Massachusetts ... We hope that, against all odds, there might be some good news, but this Kennedy clan seems to be a very troubled clan - many of us remember the very day his father was assassinated; we see it in our memories. I express sympathy to the Kennedy family."Reuse content