International Holocaust Remembrance Day is being marked around the world on Sunday while attention is being drawn to the ignorance and shocking doubts that still surround the Second World War genocide.
A survey found one in 20 UK adults does not believe the Holocaust took place, while one in 12 believes its scale has been exaggerated.
A survey of more than 2,000 people by the Holocaust Memorial Day Trust (HMDT) found almost two-thirds of respondents (64%) either could not say how many Jews were murdered or "grossly" under-estimated the number.
Candles have been lit for the six million Jewish victims killed by the Nazis while concentration camp survivors paid homage at Auschwitz in Poland.
Labour leader Jeremy Corbyn joined 200 survivors of the Holocaust and genocides to honour the millions of people who were murdered.
Britain’s Jewish community was assured it was an “intrinsic part of what makes Britain great”, at a national memorial service in Westminster.
Mr Corbyn, whose party has been dogged by allegations of anti-Semitism, arrived 20 minutes before the service began and was seen chatting and taking photos with guests.
The Remembrance Day comes amid a revival of hate-inspired violence and signs that younger generations know less about the genocide of Jews, Roma and others during the Second World War.
Since last year’s observances, an 85-year-old French Holocaust survivor, Mireille Knoll, was fatally stabbed in Paris and 11 Jews were gunned down in a Pittsburgh synagogue during Shabbat services, the deadliest attack on Jews in US history.
Human Rights First, a US organisation, recalled those killings and warned that “today’s threats do not come solely from the fringe”.
Hundreds of thousands of people, including survivors, politicians and members of the public, have gathered across Britain as Theresa May and Jeremy Corbyn both made personal tributes.
More than 11,000 activities were expected to take place, while ceremonies were being held in London, Scotland, Wales and Northern Ireland.
A national commemorative ceremony was taking place in Westminster to mark the day, which is also acknowledging the 25th anniversary of the genocide in Rwanda and 40 years since the end of the genocide in Cambodia.
Holocaust survivor Steven Frank, who was one of 93 children who survived the Theresienstadt camp in Czechoslovakia along with his two brothers, said the figures were “terribly worrying”.
His father, who helped hide Jews as part of the Dutch Resistance, was arrested in Amsterdam and taken to Auschwitz in Nazi-occupied Poland, where he was gassed on January 21 1943.
Mr Frank, 83, said he was “surprised” that the survey found as many as one in 20 people still did not believe the Holocaust took place.
He said: “In my experience, people don’t have a solid understanding of what happened during the Holocaust and that’s one of the reasons I am so committed to sharing what happened to me.
“At one of my talks, I met someone who said the Holocaust didn’t happen. The only way to fight this kind of denial and anti-Semitism is with the truth – I tell people what happened, what I saw and what I experienced.
“Education is so important. If we ignore the past, I fear history will repeat itself.”
Holocaust survivor, Rachel Levy, also shared what her family went through when they were sent to Auschwitz.
She told ITV News: "We were pushed to the left, my mother and two little sisters and little brother went to the right.
And unknown to me, because I didn't know why, that's where they went - I learned later - to the gas chambers. "
HMDT chief executive Olivia Marks-Woldman said: “The Holocaust threatened the fabric of civilisation and has implications for us all. Such widespread ignorance and even denial is shocking.
“Without a basic understanding of this recent history, we are in danger of failing to learn where a lack of respect for difference and hostility to others can ultimately lead.
“With a rise in reported hate crime in the UK and ongoing international conflicts with a risk of genocide, our world can feel fragile and vulnerable. We cannot be complacent.”
Karen Pollock, chief executive of the Holocaust Educational Trust, said: “We know that education is vital in the fight against ignorance and hate.
“Whatever the statistics, one person questioning the truth of the Holocaust is one too many and so it is up to us to redouble our efforts to ensure future generations know that it did happen and become witnesses to one of the darkest episodes in our history.”
Communities Secretary James Brokenshire, who will be speaking at Sunday’s event, said: “It remains essential now as ever to remember the Holocaust, to understand why it happened and to learn the stark lessons it gives.
“We must never forget where hatred and bigotry can lead.”
He added: “This Government is clear that anti-Semitism has no place in our society and we all have a role to play in confronting hatred and extremism whenever and wherever it occurs.”