President Barack Obama has led tributes from across the political spectrum to Senator Edward Kennedy following his death at the age of 77.
The senator, a leading member of one of America's most fabled political families, had been suffering from brain cancer. His death was announced in a family statement today.
"Michelle and I were heartbroken to learn this morning of the death of our dear friend, Senator Ted Kennedy," Mr Obama said.
"For five decades, virtually every major piece of legislation to advance the civil rights, health and economic well being of the American people bore his name and resulted from his efforts," Mr Obama said in a statement from the Massachusetts island of Martha's Vineyard where he is on holiday.
"I valued his wise counsel in the Senate, where, regardless of the swirl of events, he always had time for a new colleague. I cherished his confidence and momentous support in my race for the presidency.
"And even as he waged a valiant struggle with a mortal illness, I've profited as president from his encouragement and wisdom," said Mr Obama of the man who endorsed him for the Democratic presidential nomination.
"An important chapter in our history has come to an end. Our country has lost a great leader, who picked up the torch of his fallen brothers and became the greatest United States senator of our time," Mr Obama said.
Mr Kennedy, who had been battling brain cancer for more than a year, died late on Tuesday at his family compound in the nearby Cape Cod town of Hyannis Port.
White House officials said Mr Obama was told shortly after 2am, local time, of Mr Kennedy's death and spoke with the late senator's wife, Victoria, about 25 minutes later.
"We've lost the irreplaceable centre of our family and joyous light in our lives, but the inspiration of his faith, optimism, and perseverance will live on in our hearts forever," the Kennedy family statement said.
"He loved this country and devoted his life to serving it. He always believed that our best days were still ahead, but it's hard to imagine any of them without him."
Paying tribute, Democratic Speaker of the House Nancy Pelosi said: "Rooted in his deep patriotism, his abiding faith, and his deep concern for the least among us, no one has done more than Senator Kennedy to educate our children, care for our seniors, and ensure equality for all Americans."
US Senate Majority leader Harry Reid said it was "the thrill of my lifetime" to work with Mr Kennedy. "He was a friend, the model of public service and an American icon. As we mourn his loss, we rededicate ourselves to the causes for which he so dutifully dedicated his life.
"Senator Kennedy's legacy stands with the greatest, the most devoted, the most patriotic men and women to ever serve in these halls . . . The liberal lion's mighty roar may now fall silent, but his dream shall never die," Mr Reid said.
Republican Senator Orrin Hatch of Utah said that America had lost "a great statesman, a committed public servant, and leader of the Senate," adding that he had also lost "a treasured friend" and a "larger-than-life United States senator whose influence cannot be overstated".
California Governor Arnold Schwarzenegger, who is married to Mr Kennedy's niece Maria Shriver, said the late senator was known worldwide as champion of social justice and a political icon.
Former first lady Nancy Reagan described Mr Kennedy as "a dear friend". She said: "Given our political differences, people are sometimes surprised by how close Ronnie and I have been to the Kennedy family.
"In recent years, Ted and I found our common ground in stem cell research, and I considered him an ally and a dear friend. I will miss him."
Reuters