They never spoke, they were playing instruments that are little known to British audiences, and they only performed one instrumental piece, which lasted for more than an hour. But this was an intense, memorable and almost exhaustingly emotional set.

Kayhan Kalhor is one of Iran's greatest musicians, an exponent of the kamancheh "spike fiddle", and he was joined by Ali Bahrami Fard on the bass santur – a hammered dulcimer – for their latest version of I Will Not Stand Alone, a largely improvised piece that was influenced by the mass protests that accompanied the 2009 re-election of the former Iranian president Mahmoud Ahmadinejad. Kalhor has since moved to the US.

The duo performed, kneeling, on a low platform covered in an exquisite Persian carpet. The piece began with a sad, sturdy theme from the santur and then the brooding, mournful kamancheh joined in, first plucked, then bowed. From then on, the music, and mood, constantly changed. There were patches of unexpected silence, followed by sudden, furious fiddle lines and improvisation in which Kalhor played with his eyes shut, grey hair flopping over his face. Then the mood altered again, with exuberant, rapid-fire playing as the two musicians swapped solos almost by intuition. This was followed by yet another switch, to a more lyrical and gentle passage, before this extraordinary piece reached an upbeat and defiantly cheerful conclusion.

The Iranians were rightly billed as headliners of this Songlines Encounters festival show, but they came on first, which was tough on the excellent Çiğdem Aslan, who had to follow them. She is a Turkish Kurd based in London, lead singer with She'Koyokh and a soloist specialising in rebetika. Her impressive set included often jaunty, sad-edged songs from Turkey, Greece and Kurdistan, but inevitably sounded easy-going after the emotional onslaught from Kalhor and Fard.