"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," the statement said.

Kennedy was one of the politicians who endorsed Obama, the then first-term senator from Illinois, in the early phase of the presidential campaign in 2008, and he presented himself several times at Obama's rallies.

Obama ordered that the flag of the United States shall be flown at half-staff at the White House and upon all public buildings and grounds to mourn the Massachusetts senator.

Some former US presidents also joined in mourning him.

Siblings Eunice Kennedy Shriver (L), Jean Kennedy Smith (C) and US Senator Edward Kennedy (D-MA) take the stage for the dedication of the Rose Fitzgerald Kennedy Greenway in Boston, Massachusetts in this July 26, 2004 file photo. [Xinhua]

Jimmy Carter, who beat Kennedy in the 1980 Democratic presidential nomination, called his old rival "an unwavering advocate for the millions of less fortunate in our country."

Former President George H. W. Bush issued a statement on behalf of himself and his son, former President George W. Bush, expressing sympathies from members of the Republican Party.

"Kennedy was a seminal figure in the U.S. Senate -- a leader who answered the call to duty for some 47 years, and whose death closes a remarkable chapter in that body's history," Bush said.

In Britain, where Kennedy received an honorary knighthood from the Queen in March for his services to the US-British relationship and to the Northern Ireland peace process, Prime Minister Gordon Brown said Kennedy "will be mourned not just in America but in every continent."

Crowning him the "Senator of Senators", Brown said that he was proud to have counted him as a friend.

In Germany, Chancellor Angela Merkel said "Germany and Europe have lost a good and treasured friend."

She lauded Kennedy's "convictions and steadfastness" in advocating justice and peace, saying he was "one of the most prominent politician in American history."

In South Africa, President Jacob Zuma said Kennedy was "a comrade and a friend" of his country "in the fight for liberation".

"We grieve the loss of this great man but the legacy of his contribution to this world will continue to serve as a sort of hope to us," Zuma said in a statement.

Kennedy will be buried at Arlington National Cemetery in Virginia on Saturday, being laid to rest next to President John F. Kennedy and Senator Robert Kennedy.

His death came just weeks after that of his sister Eunice Kennedy Shriver, founder of the Special Olympics, on Aug. 11.

(Xinhua News Agency August 27, 2009)