The artist, best known for the song Let Her Go, returned to his busking roots to raise awareness of the work of The Big Issue magazine, which helps the homeless.
The singer said: "I don't need to play on the street with my guitar case out to try and live any more, so it's the least I can do to try and help Big Issue vendors across the country."
Hundreds of people were at the Royal Concert Hall to listen to him. Fans sang along to some of his biggest hits, such as Holes and Feather on the Clyde, which was written about Glasgow. He also performed songs from his new album Whispers, which is out on June 9. The singer said the last time he busked in the city only 14 people watched him.
Alex Carid, 23, said: "I was shopping with my friends and once we found out he was there we just had to go. He was fantastic. I loved every minute of it."
David Graves, 24, said: "His songs are so catchy, people were loving it. It's so great that he's doing all this."
Pictures: Colin Mearns