LONDON — A founder of the climate activism group Extinction Rebellion apologized on Thursday for the “crass words” he used to describe the Holocaust as “an almost normal event” and just another ugly episode in human history.

The founder, Roger Hallam, said in an interview with the German weekly Die Zeit that among various genocides that had occurred in previous centuries, the Holocaust, in which the Nazis killed millions of Jews during World War II, was not that unusual.

“The fact is that in our history, millions of people have been regularly killed under dire circumstances,” Mr. Hallam, who is British, said in excerpts published online on Wednesday. “To be honest, you could say: This is an almost normal event.”

Mr. Hallam’s remarks were made ahead of the German publication of his book, “Common Sense for the 21st Century.” In the book, which was published in Britain in September, he draws parallels between the Holocaust and the threats posed by the climate crisis.