Survivors of the Auschwitz death camp have joined the Queen and Tony Blair at a service in London to mark the 60th anniversary of its liberation.
The Queen met Auschwitz survivors at a reception at St James's Palace
The event at Westminster Hall was attended by hundreds of survivors and veterans of World War II.
Prime Minister Tony Blair said victims of the Holocaust must not be forgotten.
"We must never dishonour their memory by allowing the ugly poison of racial prejudice and hatred to hold sway again," he said.
He described the death inflicted by the Nazis as "an industry, not just the destruction of human life, but of human essence, done with a barbarity we can scarcely contemplate".
"This was no natural disaster, no act of God, but an act of deliberate, calculated evil such as humanity never in its existence knew before, and let us pray, never knows again," he told those gathered at the hall, in the Houses of Parliament.
Chief Rabbi Dr Jonathan Sacks said memories of the Holocaust must be passed on to children.
"Today we remember the victims of the greatest crime of man against man, and weep for a murdered generation - the young, the old, the innocent, the million and a half children gassed, burned and turned to ash because they were different," he said.
"We can't bring the dead back to life - but we can fight for the sanctity of life."
Maj Dick Williams, 84, was one of the first Allied soldiers to enter the Belsen camp on 17 April, 1945.
"I could not believe what I was seeing - the horror that was there.
"You had to pick your way through the camp because of the people who had died, some hanging from the barbed wire," he said at the palace reception.
Earlier, the Queen and Prince Philip spent more than an hour talking to survivors and veterans at a private reception at St James' Palace, among them Martha Grunwald, who turned 85 on Thursday.
Mrs Grunwald was 25 when she spent four months in Auschwitz, and lost three family members in the camp, including her mother.
"She was 55 years old, and they wanted to take her away when we arrived but I said she could walk and so she was allowed to stay.
"We used to pinch her face to give her rosy cheeks so she would not be selected, but in the end she was."
Lena Lakomy, 82, who is thought to have been the longest-serving prisoner at Auschwitz also attended.
She entered the camp in February 1943 and was liberated in May 1945.
She said it was important to remember the Holocaust: "They say 35% of people don't know about Auschwitz. I can't believe it.
"I want people to know not to be frightened of the word Auschwitz. People should know more about it."
The Queen lit the first of 60 candles in Westminster Hall.
The Nazis' final assault on the Jews from 1933-1945
Estimated 15m civilians killed by regime
6m Jews murdered
1942: Gas chambers built at Birkenau concentration camp, mass transports begin
Majority who arrive gassed immediately
About 900,000 gassed at Birkenau
Over 1.1m died at Auschwitz-Birkenau and its sub-camps
1m of them were Jewish
Also at the ceremony, grandchildren of the survivors read out the names of 3,000 of their relatives who died at the hands of the Nazis.
Archbishop of Canterbury Dr Rowan Williams and Home Secretary Charles Clarke were also present.
Altogether some 15 million civilians are thought to have been murdered by the Nazi regime, some six million of whom were Jewish.
The memorial service in Auschwitz itself was attended by leaders of 37 countries, including Presidents Jacques Chirac of France and Vladimir Putin of Russia.
Foreign Secretary Jack Straw and Prince Edward led the UK delegation, including three British survivors.