US President Obama has paid tribute to the American folk singer and activist Pete Seeger, who has died following a short illness at the age of 94.
"Once called 'America's tuning fork', Pete Seeger believed deeply in the power of song," said Mr Obama.
"But more importantly, he believed in the power of community.
"To stand up for what's right, speak out against what's wrong, and move this country closer to the America he knew we could be."
Seeger died at New York hospital, his grandson said.
His songs included Turn! Turn! Turn! and If I Had A Hammer.
In 2009, he was at a gala concert in the US capital ahead of President Obama's inauguration as president.
In his tribute, the president praised Seeger's activism.
"Over the years, Pete used his voice - and his hammer - to strike blows for worker's rights and civil rights; world peace and environmental conservation. And he always invited us to sing along.
"For reminding us where we come from and showing us where we need to go, we will always be grateful to Pete Seeger. Michelle and I send our thoughts and prayers to Pete's family and all those who loved him," he added.
Seeger gained fame in The Weavers, formed in 1948, and continued to perform in his own right in a career spanning six decades.
Renowned for his protest songs, Seeger was blacklisted by the US Government in the 1950s for his leftist stance.
Denied broadcast exposure, Seeger toured US college campuses spreading his music and ethos, later calling this the "most important job of my career".
He was quizzed by the Un-American Activities Committee in 1955 over whether he had sung for Communists, replying that he "greatly resented" the implication that his work made him any less American.
Seeger was charged with contempt of Congress, but the sentence was overturned on appeal.
He returned to TV in the late 1960s but had a protest song about the Vietnam War cut from broadcast.
British singer and left-wing activist Billy Bragg who performed with Seeger on several occasions called the singer "hugely encouraging".
"He was a very gentle man and intensely optimistic," he told the BBC. "He believed in humanity and the power of music to make a difference, not to change to the world.
"I performed at his 90th birthday and the fire was still there."
Seeger became a standard bearer for political causes from nuclear disarmament to the Occupy Wall Street movement in 2011.
In 2009, he was at a gala concert in the US capital ahead of Barack Obama's inauguration as president.
His predecessor Bill Clinton hailed the musician as "an inconvenient artist who dared to sing things as he saw them.''
Other songs that he co-wrote included Where Have All The Flowers Gone, while he was credited with making We Shall Overcome an anthem of resistance.
Turn! Turn! Turn! was made into a number one hit by The Byrds in 1965, and covered by a multitude of other artists including Dolly Parton and Chris de Burgh.
Seeger's influence continued down the decades, with his induction into the US Rock and Roll Hall of Fame in 1996, and he won a Grammy award in 1997 for best traditional folk album, Pete.
He won a further two Grammys - another for best traditional folk album in 2008 for At 89 and best children's album in 2010.
He was a nominee at Sunday night's ceremony in the spoken word category.
He was due to be honoured with the first Woody Guthrie Prize next month, given to an artist emulating the spirit of the musician's work.
Mark Radcliffe, host of BBC Radio 2's Folk show, paid tribute, saying: "Pete Seeger repeatedly put his career, his reputation and his personal security on the line so that he could play his significant musical part in campaigns for civil rights, environmental awareness and peace.
"He leaves behind a canon of songs that are both essential and true, and his contribution to folk music will be felt far into the future."
Seeger performed with Guthrie in his early years, and went on to have an effect on the protest music of later artists including Bruce Springsteen and Joan Baez.
In 2006, Springsteen recorded an album of songs originally sung by Seeger.
On his 90th birthday, Seeger was feted by artists including Springsteen, Eddie Vedder and Dave Matthews in New York's Madison Square Garden.
Springsteen called him "a living archive of America's music and conscience, a testament of the power of song and culture to nudge history along".
His other musical output included albums for children, while he appeared on screen several times as well.
A reunion concert with The Weavers in 1980 was made into a documentary, while an early appearance was in To hear My Banjo Play in 1946.
The band, who had a number one hit with Good Night, Irene in the early 1950s, went their separate ways soon afterwards.
Seeger's wife Toshi, a film-maker and activist, died aged 91 in July 2013. They leave three children.