They came to bear witness one last time. Exactly 75 years after the Red Army’s liberation of Auschwitz, those who had seen humanity’s descent into hell returned to speak of it while they still could – and with a new, double urgency. They testified like people running out of time, aware that their own mortality is pressing in on them – and alarmed that the world needs to hear their message now more than ever.

There were fewer than 200 of them at Monday’s international ceremony, confirmation that their ranks are becoming thinner and more frail with the passage of time. Feted as honoured guests in the place where once they were reviled inmates, even together they filled just a few rows. They were at the front of a vast marquee, large enough to house the iconic “gate of death” through which those men and women – almost all mere boys and girls at the time – had passed when it was not theatrically lit to make a spectacular backdrop for television, but when it had the power to terrify.

Present were the crowned and uncrowned heads of Europe, kings and princes, presidents and prime ministers. And yet in this ceremony the dignitaries stayed seated, compelled to listen rather than to speak, as the floor was handed to men and women in their 90s, who spoke in the manner of people handing a scribbled note through the window of a departing train – with an urgency that bordered on desperation.

Part of that was the simple knowledge that this was almost certainly their last such ceremony of remembrance: if they didn’t speak now, they may not get another chance. But part of it, too, was the belief that the world has changed, even in the five years since they last convened on this spot.

And so they talked of the slaughter they had witnessed, the organised attempt to eradicate the Jewish people, an effort that drenched the soil with blood and choked the air with human ash. Now Auschwitz-Birkenau is a museum, with tour parties and snack bars. But they knew this place when it throbbed with the business of genocide, when it measured 15 sq miles, a city of death roughly the size of Lincoln: a period of intense industry that left 1.3 million men, women and children dead, 90% of them Jews.

Marian Turski delivers a speech during the ceremonies. Photograph: Kacper Pempel/Reuters

It was difficult for them to speak. Elsa Baker, there to represent the Sinti and Roma people – 23,000 of whom were inmates of Auschwitz’s “Gypsy camp” – apologised that, because she was blind, someone would have to read her speech on her behalf. Batsheva Dagan, aged 95, broke off from her own speech to ask for a glass of water – a simple human need that would have gone unanswered when she was in this place eight decades ago.

For her, and for others, there was still anger that needed to be voiced – not directed at the German architects of the plan to eliminate the world’s Jews, but at a world that had stood by and let it happen. “Where was everybody?” asked Dagan. “Where was the world, who could see everything and yet did nothing to save all those thousands?” She was interrupted by applause.

But it was Marian Turski, 93, who spoke for so many of his fellow survivors when he issued what amounted to a final warning to a human race that will soon lack first-hand testimony of the depths to which humankind can sink. He explained that “Auschwitz did not fall from the sky” but was the destination reached after a thousand smaller steps, each one stripping a single minority of its dignity and humanity. After the Shoah, Turski said, “The 11th commandment is: thou shalt not be indifferent.”

This was no mere platitude, no mere parroting of “never again” banalities. It was far more pointed than that. “Do not be indifferent when any minority is discriminated against,” he said, making clear that he included eastern Europe in this injunction. It sounded like a rebuke to Viktor Orbán of Hungary, seated in the second row, as well as his own government in Poland, along perhaps with that of Donald Trump’s United States. “Democracy hinges on the rights of minorities being protected,” Turski said, lest there be any doubt.

Attendees arrive to put candles at a memorial site at the Auschwitz camp. Photograph: Czarek Sokołowski/AP

There was a similar, barely coded meaning to his exhortation against indifference “when you see that the past is stretched for current political needs”, surely a rebuke to a Polish government that made it illegal to accuse Poland of complicity in the Nazi crimes that were committed on its soil.

But it was the fear that the moral force of Auschwitz may be fading that animated so many of those who had returned to the place that still haunts their dreams. They know that a postwar world order constructed in the shadow of Auschwitz – an architecture of international institutions and supposedly universal human rights – is in peril, the conscious and deliberate target of populist nationalist governments in Europe and beyond. More simply, they fear that the taboo on bigotry and prejudice – deemed unacceptably toxic after the liberation of 1945 – is receding. Human rights lawyer Adam Wagner calls it “the dying of the light”, the notion that the once-chastening shadow of Auschwitz is getting shorter and thinner.

She didn’t put it that way, but one who shared that anxiety was Renee Salt, a 90-year-old Londoner, wrapped up in coat and scarf, but in the Auschwitz of 1944 a 15-year-old girl who had only a “a pyjama jacket” to shield herself from the bitter Polish cold. She lost her father the moment she reached the camp: she remembers him jumping off the train just before she did, but after that he “disappeared into thin air”.

Renee Salt. Photograph: Aleksandra Szmigiel/Reuters

This was not Renee’s first trip back. She has made the trek to Auschwitz often, despite admitting: “I can’t even begin to tell you how much I hate it.” Before every visit, including this one, “I get so nervous, it feels terrible.” She gestured towards her abdomen, to indicate the knot in her stomach. Asked if this would be her last visit, she nodded before saying: “At my age, I don’t make any plans.”

And yet she forced herself to return, pulled there by the same sense of obligation that tugged at so many of the remaining survivors. “I feel I have to. If I survived, I need to come back to show people what happened.”

She shares the fear of many survivors too that once they have gone, once living memory becomes dead history, the power of the Shoah will fade, that people will forget its agony and its lessons. They worry that even though the attempted elimination of the Jews was one of the most documented crimes in human history, that it will lose its force once there are no longer living, breathing human beings around to say: “I was there.”

But she looks around today’s world and worries that all the testimony and all the evidence are not working anyway. Recent acts of terror, murderous acts of violence, including against Jews, have left their mark on her. “I know that the world hasn’t learned from our experience,” she said. “It’s forgetting.”