War and peace are matters for the prime minister alone, the junior Foreign Office minister Ben Bradshaw - no constitutional authority he - claimed recently. Tony Blair must wish the hapless Bradshaw was right.

For the people who put him in office are taking hold of the debate over whether or not to back George Bush's planned aggression against Iraq, as they have not done on a matter of foreign policy for many years.

There is a sense that the issue hangs in the balance and that the voices of public opinion can determine Britain's part in the outcome. In the run-up to Saturday's anti-war demonstration in London, likely to be the biggest such protest for more than 30 years, speakers from the Stop the War Coalition have been at scores of meetings in all parts of the country - hundreds in a hall and hundreds more outside in Bradford this week, a huge gathering in fascist-plagued Burnley, a community center packed to overflowing in Watford, the regional council of a major trade union.

There is not just opposition to the prospect of war - there is a boiling anger. It is expressed most sharply, in my experience, among two groups of people: the Muslim community and the membership of the Labour party, the latter including some who have made a public point of resigning their membership.

British Muslims have engaged with this campaign as never before, enraged by a "war against terror" that seems selectively to target countries in the Islamic world while ignoring other transgressors against international law, most notably Israel. It is a landmark that this Saturday's demonstration is jointly organized by the Muslim Association of Britain.

But it spreads wider. More and more trade unions are declaring against the war, from all the rail unions to the firefighters to giants of the movement such as the T&G and Unison. And Charles Kennedy has become surely the first Liberal leader to declare to his constituency that the United States is following an "imperialist" policy.

Probably the only time the prime minister can outline his Iraq policy without getting an argument is in his private meetings with Iain Duncan Smith.

The Herculean efforts of the government spin machine to at least smooth the edges of this opposition seem to be failing. Every trial propaganda kite floated in London has ended up being shot down by friendly fire from the hawks in Washington.

Blair no sooner says "act through the UN" than Rumsfeld or Cheney bellow that "the US will act alone". Each guarantee that "this is all about weapons of mass destruction" is negated on the instant by Rice or Wolfowitz declaring that "regime change at any cost" is the only objective.

That is the prime minister's problem. He is selling one policy, but the whole world can see that it is not the policy being followed by the people who matter.

And public opinion grows daily more convinced of one thing above all: the present US administration is controlled by a group of very dangerous men and women headed up by a simple-minded president.

Look, for example, at the speed and brutality of the US rejection of Iraq's offer to admit weapons inspectors unconditionally. Within 24 hours the administration went from threatening war to impose compliance with United Nations resolutions to threatening action to impede any effort at such compliance.

And last week, with world opinion in the balance over Iraq, the Bushmen unveiled their Martini doctrine of war - anytime, anyplace, anyhow. The world has been shaken more than stirred by this call to arms.

So for the most part the prime minister's dossiers and declarations of multilateral good intent are wasted words. There is a settled opposition to this war based, in large measure, on a sense that the world is sliding towards endless war and that Tony Blair is putting on the blinkers rather than searching for the handbrake.

Perhaps the prime minister would like to go further. Regime change by force in Zimbabwe? No doubt he's thought about it, what with his missionary passion for Africa.

How long will the British people themselves be left unscathed by the consequences of this? Very large numbers don't intend to sit around waiting to find out. Many will march on Saturday, but that will not be the end of it. After all, if "decent, law-abiding folk" can discuss civil disobedience and direct action the better to kill foxes, can we do less to save human beings?

Andrew Murray is chair of the Stop the War Coalition, which is organizing Saturday's march in London

apdmurray@hotmail.com

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