The 74-year-old made an appearance at the Democratic National Convention in Philadelphia to sing the 1970 gospel-inspired song, Bridge over Troubled Water - a track he wrote which was originally sung by former musical partner Art Garfunkel.

His performance was well received, however, some viewers criticised the music legend on social media, claiming he appeared unwell, and was singing out of key.

The You Can Call Me Al singer who recently hinted at retirement, has been a keen supporter of former presidential hopeful Bernie Sanders, who failed to win the Democratic nomination, as has his former partner. And they both agreed Sanders could use their song America on his campaign trail.

Simon finished his tour supporting his latest album, Stranger to Stranger, earlier this month (Jul16), and hinted in a recent interview with the New York Times at his possible retirement from showbusiness.

“Showbiz doesn’t hold any interest for me," he told the New York Times.

The singer-songwriter blamed the physical impact of touring, and having to rest for long periods between dates for his reluctance to continue.

“It’s an act of courage to let go,” Simon told the publication. “I am going to see what happens if I let go. Then I’m going to see, who am I? Or am I just this person that was defined by what I did? And if that’s gone, if you have to make up yourself, who are you?”.

Grammy award winning R&B stars Boyz II Men also entertained delegates, and singer Demi Lovato, who took the opportunity to campaign for more work to be done by politicians to combat the the stigma associated with mental illness.

"This is not about politics, it is simply the right thing to do," she told delegates, before performing her hit Confident.