Viggo Mortensen as Aragorn in Lord of the Rings. Photo / Supplied

Hollywood star Viggo Mortensen surprised viewers at a Lord of the Rings movie marathon by turning up unannounced and performing a song in elvish.

Rings fans were sitting through all three of Peter Jackson's acclaimed films at the Coolidge Corner Theatre in Boston when Mortensen unexpectedly arrived on stage, reported theonering.net.

Mortensen, who was in town to receive an award, greeted the amazed crowd and sung the song Aragorn's Coronation from Return of the King.

"I was more nervous singing that just now than I was in the studio," Mortensen said after the singalong, which included crowd participation.

Mortensen starred as Aragorn in all three of Peter Jackson's Rings films, The Fellowship of the Ring, The Two Towers and Return of the King.

Watch the footage of Mortensen's unexpected visit: