It remains the biggest demonstration in British history, hundreds of thousands of people took to the streets of London on February 15 to oppose Britain's involvement in the impending Iraq war.
Attendance estimates vary from between 750,000 to two million, and included celebrities, politicians, civil rights activists and high-profile leaders - but mostly ordinary people. School pupils came in their droves, and the Muslim community was mobilised to act.
The Huffington Post UK has spoken to marchers across the spectrum, asking for their memories of that day in February.
ANDREW MURRAY, STOP THE WAR COALITION, ORGANISER OF THE LONDON DEMONSTRATION
A former Morning Star journalist and key player in Britain's Communist Party, Andrew Murray was chair of the Stop The War coalition and one the demonstration's key organisers.
He admits he had no idea how massive the movement would become. "It did only slowly dawn on us that it would be the biggest demonstration Britain had ever seen. We had no idea, even on the day. And far more people wanted to speak at the demonstration than possibly could. There was no shortage of temperamental personalities."
The march was a logistical nightmare, with then Culture Secretary Tessa Jowell banning the march from Hyde Park, with a week to go, because of damage to the grass.
Murray described a meeting with officials and the police, "I said to them: 'I want you to consider how it is that you are going to stop more than one million people coming into Hyde Park.' The police went away into a huddle, and then said we could use the park. The establishment was certainly panicked."
Murray said he felt a "huge sense of responsibility" in the days leading up to the march. But a shock came the night before. "My 16-year-old son had been attacked on the street, and he was in hospital, mercifully not too badly hurt. That had really taken my attention."
One image stuck with him. "The whole of Piccadilly, drowned in people, but not a single banner among them. They had not come as part of a political group, or a trade union. They were just individuals, some with home-made placards.
He does feel a sense of bitterness, as well as achievement: "There is disappointment and anger, of course. We were completely ignored.
"But I think it has made politicians more wary about the future, like an attack on Iran, Syria, at least in part. Tony Blair's premiership, a man who was a talented politician, elected with an enormous majority, will now only be remembered for that."
KEN LIVINGSTONE, THEN MAYOR OF LONDON
Ken Livingstone was three years into his term as Mayor of London when the Iraq War loomed. He was independent of any political party and his staunch anti-war credentials meant he was never in any doubt as to whether to support the march.
But, he said, he did feel " a real dual responsibility, on the one hand as Mayor of London, we had to organise the police, make sure no-one would be crushed to death. Being mayor, and being part of that demo always makes for interesting conflicts. You have to make sure nothing goes wrong.
"None of my staff ever advised me against it though, they were totally committed. And Tony Blair wouldn't have wasted his breath.
"I was so desperate for us to get more people than the fox-hunting people, who marched a few weeks earlier."
"I had never seen so many people at any one time. I doubt I will again. It was so clear to people that it was fundamentally wrong," he said, recalling the day of the march. "It was the population of the UK at the time of 1066, two million people."
His speech at the march was almost overshadowed by some overzealous body guards of civil rights activist Jesse Jackson, who would speak at the demo. "As I went up to the platform to speak, a chap called Aaron Barshack ran onto the stage and shouted 'No congestion charge'.
"Because Jesse Jackson was at risk of assassination, he had a huge security presence. And they moved like lightning, picking him up and throwing him offstage. It was a slightly nervy moment for all concerned."
KEN LOACH, FILM DIRECTOR
A long-time anti-war campaigner, Loach was one of a number of well-known faces that also included celebrities like Ms Dynamite, Harry Enfield, Emma Thompson and Chris Eubank who took part in the demonstration in London.
"It was a genuinely democratic expression," he said. "There was a strong sense of comradeship, an inclusivity to the whole day. What struck you immediately was the size of it, the thousands and thousands of people."
Like many others, Loach says he is bitterly disappointed that the spirit of the march has never been recaptured and no long-term movement was formed. "I think we made a very genuine mistake. We had no organisation in place after the march finished.
"Once people had walked away we had no way of reaching them again, no way of getting back in touch with them. I think that could have been the basis for a whole new political organisation which never was.
"It was a great demonstration but it stayed a demonstration. My strongest feeling is one of great regret.
"I would hope a reawakening of that kind would be possible again. This government has crass policies but a PR man as PM. I think people feel as strongly about things like the NHS, but they don't have an outlet for their anger."
THE ETON COLLEGE ORWELL SOCIETY
It was a banner not many had expected to see on the march, painted in the art room of one of the most exclusive schools in the country.
It read: "Eton College Orwell Society. People Not Profit. Peace Not War." A contingent of 60 boys had taken the train from Eton to London to join the thousands of high school students, many of whom had travelled much further to be there.
One of the pupils leading the Orwell Society, was Ruairidh Villar, who gave an interview to The Times while on the march, telling the paper: “Prince Harry pulls my peace badges off."
Now a news agency journalist working in Japan, Villar recalled: "We were expecting a bit of negative attention. But we got so many smiles and handshakes. It was an amazing equaliser.
"Everyone was mobilised, young and old, Christian and Muslim, privates schools, state schools. For all of us, it was an eye-opener."
Dr Steven Cullen, the then Eton master who helped supervise the society, said he had found the school much more supportive of the students political activism that his son's local comprehensive.
"I think it's a mark of how confident Eton is in what it does, I think the boys would have gone anyway, so the school dealt with it intelligently."
Villar says his overriding feeling about the day, ten years on is "a combination of failure and feeling failed. We went out so idealistic about it and then, ultimately, it came to nothing."
One memory has stuck with him. "Waiting in the train station on the way back, and this woman came up to me and started shouting and saying 'Why are you protesting against this war?' She was Iraqi and she told me her husband had disappeared in Iraq.
Villar said her words have only hit home later in life. "I was out in Libya last year, and I arrived in Benghazi, on the slabs in the morgue there were old women, six-year-old girls hit by snipers.
"And now looking back to that encounter in a train station with an Iraqi woman, I remember thinking to myself, it was all so obvious, everyone was against the war, why wasn't she? I can realise now that the Eton College anti-war society didn't really know what was going on. Partly it was an escape. What it meant to Iraqis was secondary."
SALMA YAQOOB, BIRMINGHAM ANTI-WAR ACTIVIST, LATER LEADER OF THE RESPECT PARTY
The Iraq War demonstration was a pivotal moment in the political future of Salma Yaqoob, who would go on to be the leader of the Respect Party.
An anti-war activist with three small children in 2003, she organised "around 200 coaches from Birmingham" to cope with demand.
"It was so hopeful, in the run up to it, we were speaking about it in community centres, churches, schools, colleges, there was a real buzz leading up to the march.
"We all came down on the coach and I was supposed to be speaking at the march. I hadn't written anything at all, and I had to scribble it on the coach. We could see people on the motorway going down to the demonstration, waving from their coaches. It was such a wonderful, community atmosphere."
The diversity of the demonstration really struck Yaqoob. "There were so many Asian women, "aunties" we call them. They were giving out chapati rolls, handing out food. It was a real community spirit. I loved the banners which say 'Make Tea Not War'. I still have one framed in my kitchen."
Yaqoob insists that the demonstration, while it did not stop the war, played a crucial role in the perception of the British people abroad. "I still feel a huge sense of pride, my conscience is clear about doing as much as I could have done.
"The reality is that millions of peoples' lives were devastated in Iraq. I got messages from people in Iraq saying, thank you so much. They didn't feel as isolated. On a human level, that made an impact. It went some way towards showing the difference between the government and the people's will.
"I think the anti-war movement has been the best antidote to extremism, not the war on terror. It provided a constructive channel for anger."