BRITAIN/DAY OF PROTEST: British Prime Minister, Mr Tony Blair, will be nervously watching an anti-war protest through central London this afternoon which could prove the largest peace-time demonstration in the capital's history.
The organisers have revised earlier estimates for a half-million turnout upwards toward "the high hundreds of thousands".
If those forecasts are realised today's demonstration will far exceed the massive pro-countryside Liberty and Livelihood march last September and put into the shade the anti-poll tax demonstrations which contributed to the fall of Mrs Thatcher.
While no one is yet predicting the fall of Mr Blair, today's turnout in London - and at rallies and vigils across Britain - could dramatically illustrate just how dangerously exposed Mr Blair has become on the issue of an American-led war against Iraq.
Mr Blair still appears supremely confident that he and President George W. Bush can secure a second United Nations resolution authorising force, and that public opinion will turn his way if and when troops are committed to conflict. Even if highly successful, there are also doubts about the likely enduring political impact of today's demonstrations given the disparate nature of the anti-war coalition.
Conservatives as well as Trotskyites will assemble behind the banners of the main organisers - the Stop the War Coalition, the Campaign for Nuclear Disarmament and the Muslim Association of Britain.
Previous protests have also attracted support from trade unions and a wide variety or organisations from Partizans (People Against Rio Tinto), the Colombia Solidarity Campaign, the Green Party, the Arab Organisation of Human Rights, Artists and Lawyers Against War, the Campaign Group of Labour MPs, Labour Left briefing and the Communist Party of Great Britain.
Matthew Parris, the award-winning journalist and former Conservative MP, admits he will find himself uncomfortable in the company of some from the hard left, and that he dislikes "the clenched fist" of the massed crowd.
However, he will be there for the march on Hyde Park to register the point that governments contemplating war need broad and popular support, the current absence of which was also sharply underlined this week by former Conservative chancellor Mr Kenneth Clarke. And this is the point which will be troubling Mr Blair and his advisers, even as they insist the expected war is only presently unpopular.
For if the turnout is on anything like the predicted scale, then the marching crowd will not be confined to the "usual suspects" and political activists but comprise vast numbers of those "ordinary" citizens who have been telling the pollsters they do not consider the case for war made. Nearly nine-out-of-ten polled say they will not back war without UN approval.
The Embankment and Gower Street assembly point marchers will converge on Piccadilly before making their way to Hyde Park, where they will be addressed by the mayor, Ken Livingstone, former presidential candidate the Rev Jesse Jackson, Liberal Democrat leader Charles Kennedy and others.
Blur singer Damon Albarn will also be on stage, as will Ms Dynamite.
It promises to be a day of great political theatre, the scene is already set by an Oscar-winning performance from Glenda Jackson - the former actress turned Labour MP, and ex-Blairite minister now a doughty opponent of war - who this week told the Commons it was not her party she was ashamed of but her government.