I simply want to say: this war is not in my name

The march was a softly spoken warning. Next time, if there is a next time, there will be anger in the air

I was as struck by the restrained demeanour of the people who marched to Hyde Park in London on Saturday as I was by the size of the gathering. Those who had arrived together in a group quietly chatted among themselves as they strode along Piccadilly to the park. Couples and single people were often enveloped in cocoons of silence. Banners were carried, and there were snatches of music and a certain amount of rhythmic chanting, whistle blowing of course, but little discussion or argument. New arrivals asked the police which way to go and received affable responses. The political temperature was exactly in line with the weather, close to zero.

I was as struck by the restrained demeanour of the people who marched to Hyde Park in London on Saturday as I was by the size of the gathering. Those who had arrived together in a group quietly chatted among themselves as they strode along Piccadilly to the park. Couples and single people were often enveloped in cocoons of silence. Banners were carried, and there were snatches of music and a certain amount of rhythmic chanting, whistle blowing of course, but little discussion or argument. New arrivals asked the police which way to go and received affable responses. The political temperature was exactly in line with the weather, close to zero.

In Hyde Park, the speakers said what the crowd wanted to hear and received murmured approval and generally subdued applause. Charles Kennedy, the Liberal Democrat leader, tried to make flesh creep with the news that this week, in the midst of the crisis, the House of Commons would be on holiday while the "unelected" House of Lords would be sitting. This arcane point from the Westminster hothouse rendered the audience dumb with indifference.

I don't emphasise the reticence of the crowd in order to diminish the importance of the occasion, quite the contrary. The mood was well caught by numerous posters that bore the legend "Not in Our Name". This is as far from inflammatory as a slogan can be. It is courteous. It is saying "Excuse me, Prime Minister, but I don't agree". However, it has the sort of quiet insistence that implies that "Next time, I won't be so polite". And this is the aspect that Tony Blair should consider, the softly spoken warning that one disregards at one's peril. Next time, if there is a next time, there will be anger in the air.

For there is a titanic, historic struggle going on, is there not? Why would Germany, of all countries, as well as France, get into a shouting match with the US? What causes Nato meetings to break up in public acrimony for the first time in 50 years? How come the Security Council erupts into unprecedented applause when the French Foreign Minister emotionally opposes the US? Why were there marches in so many countries involving so many people?

If we seek out the immediate reasons for each event we may miss the underlying explanation. It is this: millions of citizens and many governments are saying "no way" to the doctrine of pre-emptive attack enunciated by President Bush following the events of 11 September.

The first example happens to be Iraq. But the big issue is how the US, the world's only superpower, conducts itself. What we expect is that the US will operate through international institutions such as the UN rather than going it alone, and that it will observe treaty commitments rather than repudiate them .

The use of pre-emptive strikes equals "might is right" and is anarchy. It is very surprising that we have got to this, but the rest of the world sees the US as an out-of-control giant liable to lash out indiscriminately and unpredictably. The struggle is to bring it back within the rules. Could it be that Mr Blair has come to understand the dangers of the new American doctrine, and is that why in his speech on Saturday he moved on to what he considers to be the safest ground, the moral case for removing a leader who inflicts such unimaginable and vile suffering on the Iraqi people?

In addition, I think, the sheer size of the London march delivered two further rebukes to the Prime Minister. At least, I would like to believe that this was so. The first one, if I am right, is the assertion that if Mr Blair continues to treat Parliament as a place of little importance, then we shall have to take to the streets a bit more often, and in larger numbers than we are used to doing. It is not the British way. It would, indeed, be very striking if the numbers engaging in street protest rose while turnout in general elections continued to decline.

The second matter is trust. The other day, as everybody now knows, Downing Street issued a document purporting to provide the latest intelligence briefing about Iraq that could be safely published without compromising sources. It turned out to be old material copied from an already published academic source that was then hyped up; the whole concoction was put together by junior press officers working for the Prime Minister. It was close to being a fraud on the public. It was certainly an insult. How many people were made more inclined to march on Saturday because of this incident? More than a few, I suspect.

Whenever the people take to the streets, it is important. And so it was on Saturday. Three-quarters of a million people had a message for the Prime Minister. As well as "Not in Our Name", another popular poster carried the words "Don't Attack Iraq". The significance of the event, however, extended well beyond that unfortunate country.