"I was shocked and flabbergasted," the head of Cologne's prestigious Philharmonic Hall, Louwrens Langevoort, told AFP via telephone on Wednesday.
"These were elderly concertgoers who showed no respect for the performer, or the music or their fellow audience members and noisily prevented the concert from going ahead," he said.
"If it had been young people disrupting the event, they would have been the first to complain that young people today have no manners. But it's the older people who displayed a lack of education here. We've experienced nothing like this before in the 30 years since the Philharmonic Hall was built."
The incident occurred during the hall's normally rather sedate afternoon series, "Sunday at Four", popular among Cologne's older classical music lovers.
In this particular concert Cologne's own period-instrument band, Concerto Köln, had teamed up with a rising harpsichord star, Tehran-born and London-based Mahan Esfahani.
The programme featured works by baroque composers Johann Sebastian Bach and Carl Philipp Emmanuel Bach as well as modern composers Fred Frith, Henryk Gorecki and Steve Reich.
Langevoort said that the hall was nearly full with around 1,800 people in the audience.
But things turned nasty when Esfahani began playing Steve Reich's 1967 piece "Piano Phase", originally written for two pianos, but in a special arrangement for harpsichord and tape recorder.
Esfahani gave a short introductory talk in English, during which one audience member shouted at him to "speak in German".
And then only minutes into the 16-minute piece, some audience members started slow-clapping, whistling and shouting to express their displeasure at the music.
Others stood up noisily and walked out in protest, eventually forcing Esfahani to stop, Langevoort said.
The harpsichordist, who subsequently posted on Twitter that he was "pretty sure that was the first time complete pandemonium and hostility broke out in a harpsichord concert," turned to the audience to ask them "What are you afraid of?"
Eventually calm was restored and Esfahani and the orchestra were able to wrap up with a concerto by CPE Bach.
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But the ugly incident has made waves in Germany's classical music world.
Langevoort said he has invited Esfahani to come back next year to perform the Reich piece in full, "hopefully this time without the tumult".
The western city of Cologne, which prides itself on its tolerance and multi-culturalism, was the scene of mob violence at the New Year, when hundreds of women reported being sexually assaulted or robbed by men of North African appearance.
That incident has inflamed tensions in Germany, which took in nearly 1.1 million asylum seekers in 2015, and put pressure on Chancellor Angela Merkel for her welcoming stance toward refugees fleeing war.